When I think of a band that takes themselves too seriously I'm usually not thinking about pretentious prog or something, but usually ridiculous black and power metal bands who are completely dead serious about their image/lyrics/music. Like in Headbanger's Journey, the interview with Mayhem where the guy is like "we're the best fucking band in the world and anyone who doesn't think so can fucking die".
Yeah, that's the only time I wouldn't argue with someone calling a band too serious. Provided they band themselves are actually being serious (they rarely are).
People who say Pain of Salvation take themselves too seriously haven't heard Disco Queen, Sleeping Under The Stars, or, god forbid, Bonus Track B. Even in Be, the most ridiculous, over-the-top album ever, the Mr. Money parts seem very tongue-in-cheek (until Iter Impius). And when it's just aimed at Daniel Gildenlow, I don't know, while he can come off as an arrogant prick from time to time, I don't think he takes himself too seriously, he's just confident in his music, and there's nothing wrong with that.