Comics (be them web or not) are both a visual and a written medium, though not always (you can call a comic without words a mute comic, but you cannot call a comic without pictures a comic at all). Still, this means the effort put into making them must be at least doubled, for the good comic creator is both a good artist and a good writer, or at least find someone else to do the part he is not good at for him. Either way, it takes work, and those who actually have a commitment to improve on either or both of those areas deserve respect.
On the other hand, those who simply are not interested at all do not. And that's why we hate lazy artists/writers.
Again, comics involve art and writing. Bad art might be redeemable by awesome writing, but good art makes a lousy job supporting bad writing. Still, either way you have a bad work, and bad works must improve until they're good, and then keep improving. Ideally, an artist should never stop improving and trying to be better, or at least try new effects and points of view.
Still, many webcomics are badly regarded due to their authors' poor attitude towards this end. They've found an easy way to make comics, have reached an uncanny level of success and stopped there. In short, they are lazy.
Lazy Art comes in many flavors, but usually it's marked by ease-increasing techniques which allow the artist to do his work easier and/or faster, usually sacrificing quality. These techniques are not bad in and of themselves, but when used mercilessly, tend to feel worse and worse.
Minimalism is one common trait of lazy art. Under normal conditions, an artist is allowed to not draw what is not important. Backgrounds, for instance, are always a rich addition to a scene, but they might also be distracting. Some minimalist styles also neglect details of character depiction, such as reducing people to floating heads, stick figures or geometric shapes barely recognizable as human figures. If used well, this may form a style. When used poorly -- as it generally is -- it shows that the artist just doesn't care.
Copy-pasting is another overused technique. The objective is to expedite the production of the comic. If your scene occurs on the same space during the whole comic, copy and pasting the background is understandable (though it might be better to offer different angles or ambiance, if possible). If one character will remain motionless for several panels, it's just normal to copy and paste them several times (again, not desirable), but a lazy artist will turn copy/pasting into his personal style (or lack of, thereof).
Lazy Writing also comes in many flavors, but they're all bad. One of them is not all bad, but should be used in moderation, much like pepper, just to cling onto the culinary metaphor a bit more.
The lazy writer will usually resort to unoriginal jokes; those that are flying all over the internet for the last couple of weeks, months or even years, some of them even older than the internet itself. Alternatively, he might make an original joke and then bash it again and again, until it turns into a bloody and disgusting pulp of the originally funny joke, in what might be called a running gag, except it fails to become funnier with each iteration.
References are also a full plate for the lazy writer. A reference is directly aimed towards the fan, most of whom don't really care if the joke itself is good, but that his preferences are shared. Many times the laughter is less a "ha ha, this is really funny!", but instead "ha ha, it's X, that game/film/book I like!" Even when humor is attempted, it's usually a cliche such as "how X would happen if it was in the real world" or "X meets Y".
And, finally, maybe the tritest of them all, the non-visual joke turned visual. Simply, it's when the writer makes a joke and the art doesn't add anything to it. You usually end up with the trite "two people talking" layout, which is greatly associated with "two gamers in a couch" webcomics. Try this one day: pick one of those comics and eliminate the art. Attribute each part of the dialogue to one letter (A, B, C, Pi...) and see how much humor remains. Great chances are the art is quite superficial.
Why is it Bad
Again, improvement keeps art fresh, and freshness is always good. An artist that just stops at his "local peak" will never know if there's a higher peak to be reached. He'll never get better. He'll just stay on the same place, basking in his internet glory and the praises of his inane and easily replaceable fanbase while his work stagnates.
Boy, this is so easy...
Seriously, Randall Munroe is quite a character. He does an entire comic with stick figures, and that's what he's famous for. But did you know he can actually draw...
And yet, all you'll see these days are stick figures on very minimalistic backgrounds, with an occasional display of coloring skills and perspective, or an occasional non-stick-figure vagina. I'm not kidding, that's why I won't provide a link.
Anyway, this only proves that Randall is a lazy artist. He insists on keeping his art to a minimum, despite the fact he actually makes money with his comics.
Cyanide and Happiness
Also referred to as "xkcd's retarded brother", Cyanide and Happiness has never displayed any degree of skill beyond their weird stick... dolls? Maybe the best artist among the four is Kris, but that's only because he at least tries drawing things other than badly drawn human beings or work some shading with his shoddy coloring skills.
It still remains bad, and especially lazy. Not to mention the addition of visuals to such crude jokes do no service for the world, at all.
The king of copy/paste himself, the master of joining parts of pre-made bases to make full comics, all paired with blurred images from Google Image Searches... Mr. B^Uckley himself!
I don't think there's much more to be said on that... seriously, we have an entire review dedicated to this shoddy example of bad webcomic crafting, so let me just summarize: basing your technique on copy/pasting is bad. Really, really, bad, because it limits your art and what it can convey, and also looks cheap.
Surviving the World
Among all the different brands of lazy we see on this wiki, none have ever been so brazen, so entitled, and so pretentious as Surviving The World. This "comic" is comprised of nothing more than some guy taking pictures of himself in a labcoat, pointing at a chalkboard... and that's it. And apparently, this is his idea of a "webcomic" because it qualifies as a photocomic. Unfortunately, photo comics require actual skill at photography, which is something author Professor Dante Shepherd has yet to demonstrate.
I never thought I'd ever have to type this, but no, taking a single picture of yourself in the same place over and over again, with absolutely no regard for framing, graphic design or photo correction does not even register as a webcomic. If that's the case, then every cretin who's ever took a picture of his wang to send to an underage girl could be considered a "photocomicker". Try explaining that one to Chris Hansen.