Hm. I'm not really a fan of certain popular comics and I don't really hide it or advertise it. Do I think other people should not read them? Not at all! If you enjoy popular-comic-here by all means, knock yourself out. I think the Oatmeal is pretty funny even though it's a bit lower than my usual standards... being primarily a graphic novel/srs business comic reader.
There absolutely are legitimate reasons for not liking certain popular comics. Penny Arcade may have been groundbreaking once, but now it's just generic commentary on games I'll probably never play, and hence the references fall short. XKCD is artistically unstriking and is intended for office drones who'll print out the comics and send them to their buddies, really. But they both appeal to very wide groups of people... gamers, people who like math and science, etc. I'm not in that demographic, but even though I'm not, I'm not going to say "and you should never ever like this comic". If you do, great! I don't.
Which leads me to a point:
I hate when people say "you have to like this comic/book/movie/series/whatever", generally "in order to keep geek cred" or "because it is the best thing ever and OMFGFROTHATMOUTHNOONECANHATEIT". I hate this as much as I hate people saying "no one should like this because it's trash". I respect that there are people who like Twilight. I am just not in the demographic.
But see, this bugs me. I like Lord of the Rings but I wouldn't force it down someone's throat, and frankly I don't think people should try copy writing Tolkien's style, they should make their own. I don't really like Firefly and I'm sick of people telling me that I really really should watch and love this series.
There's nothing wrong with genuine constructive criticism of popular works. For example, I don't like Twilight because I find the main character to be an insipid waste of time and ink, but rather than posting "Everyone needs to hate Bella" I would rather post that I find the generic every-girl template to be unoriginal and the fact that this book attempts to push the clumsy, selfish, self-hating template as "this is being a good normal girl" at me puts me off. Other people love it BECAUSE of the every-girl template. But I read books where girls kicked ass, not flung themselves at male-hero to be rescued. This constructive criticism is not a bad thing; because you're using a popular work many people will be familiar with, they'll be able to understand and visualize the point. For a budding creator, or an ongoing creator or even a well established creator, the knowledge that a generic every-girl template narrator is grating to some readers might be just what they needed to know when writing their next thing.
Posting TWILIGHT SUX AND NO ONE SHOULD EVER READ IT EVER will achieve nothing but establish you as a frothing-at-the-mouth hater. This is like the relationship between my sister and Justin Beiber. She absolutely hates his music and doesn't think anyone should listen to his not-music ever. I think she's just out of the demographic his music appeals to, though maybe not that much because she's a Lady Gaga fangirl.
So TL;DR: Ripping on comics doesn't get anywhere. Constructive criticism, however, can be helpful for everyone! Yay, constructive criticism! Even if I don't like Penny Arcade I respect that some people do and find it rater irritating when people post about how they hate popular things.