I would say that in some ways comic books are sexist, but not entirely. Comic books do not feature women in their comic books as much as they feature men and that is apparent to people who do not even read comic books. Even when women are featured as super heroes in comic books they are portrayed as weaker humans. For example in the Fantastic Four the Invisible Girl is called the Invisible Girl rather than the Invisible Woman making her seem younger and inferior to her super hero counter parts. Also the Invisible Girl can only turn invisible, that is her only power. This is also another example of her being inferior to her super hero counter parts. All of the other members of the Fantastic Four have powers that are so much more useful against the villains they fight.
There is an explanation for the lack of women figures in comic books however. The fan base of comic books is made up with mostly men. Authors cater their writing towards their readers so why would they write about women super heroes as much as they do men? Men are going to want to read about men fighting and saving the world and not women because most of the times women are not associated with fighting and things of that nature. Although this is not a proper excuse for the lack of women figures in comic books it is one that makes sense.