Personally, I can't stand a comic that takes itself seriously all the time. If nothing else, it doesn't feel real to me. Real life has a mixture of humor and drama, and even the most horrible events can have a hint of humor to them, or at the very least I can understand the surviving characters trying to relax after a traumatic experience with something humorous. There's a way to blend the two, and when it's done sloppily it's a real problem.
But maybe I'm just reading what you meant wrong. Do you have an example of what you're talking about?
I think the problem with injecting humor into otherwise extremely serious stories (to me, anyway) is when it seems completely out of place. If it's not done carefully, it can feel "cheap", like the author just stuck it in to get a rise out of the audience or to satisfy their craving to stick in a personal joke where it didn't really belong. If a character delivering the joke has proven themselves otherwise not prone to making them, it just makes it seem even more awkward.
That said, I don't think humor is a bad thing at all in comics, and I prefer the ones I read (even serious ones!) to make me smirk, or even laugh out loud, once in a while. If it's one of those stories with absolutely no humor whatsoever and it's all doom and gloom, I tend to get bored with it after a while, unless there's a pretty damn good reason that there's not even the slightest hint of humor in any of the characters. (The same can be said for all-humor comics, though (save gag-strips, obviously (wow, parentheseis inside of parenthesis, wooo)). I guess what I'm saying is I prefer a mix of both. But that's just personal preference, so I think I'm getting off topic.)