it's 3 in the morning but why the hell not. to preface I am not a woman, so I cannot speak on that experience, however I would like to thinK I have a gist of an idea.
, is the tl;dr, non SJW answer that even reasonable people should be able to comprehend.
In detail, there are several reasons,
1. contributes to the fetishization of a certain type of woman/body type (in over consumption/exposure). In comics, basically 95% of the women are well endowed women with conventionally attractive proportions. Yes, I understand men are beefcakes as well, but men are not fetishized as much as women. Nor does the fetishization of men have any sort of significant societal consequence.
2. mostly men dictating the creation, drawing, and portrayal of these women. Feeds into the idea that women's primary function is serve as consumption for men.
now before you respond back to me all angry, please understand that 1. I like cheesecake, 2. not all feminists share the same ideas about cheesecake, 3. some probably like cheesecake as well. I understand that comics unrealistically portrays everybody, and a lot of the time people just like to draw sexy women.
Remember that reasonable feminists probably aren't mad at the sexy by itself, they're mad that women are sexy first and devoid of character/plot relevance second. They're mad that every depiction of a woman has to be conventionally sexy because, again, it feeds into the idea that a woman's value is only based on her level of attractiveness.
And I wouldn't reduce the complaints to just "ugly women". The way most women are drawn in comics just isn't possible by the vast majority of women even if they were physically fit. Most women aren't gonna have the ass and tits of Spider-Woman or Scarlet Witch or WW without body modifications. That's just the truth about anatomy.