Well, of course there's sexism, or at least stereotyping of women, in comic books. Creators are human beings, and for the most part have historically been men--why should they be expected to be particularly enlightened? They're getting paid to work in an industry that focuses on fiction, fantasy, etc.
In general they're going to write/create women characters that conform to their own viewpoints/fantasies, whether it's 1960s women who loved shopping and were often dependent on men (original 1960s depictions of Sue, Jean, etc.) to today's women can do everything! while wearing skintight/very little clothing! mode. In other words, for any given era it's men's stereotyping of that generation's "perfect woman."
And let's face it, it's also what the comic book-reading target audience wants, and the target audience for superheroes is still largely composed of men. One need only take a look at, say, the CBR Community Cover Contest and note the frequent posting of cheesecake (putting it mildly) covers that blatantly objectify women and reduce women to sexual fantasy stereotypes.