I think that was just a joke, and an obscure reference to Rocko's modern life people already pointed out. I don't think they're implying Rad in his underwear is sexy
And I don't necessarily agree with tumblr but basically it's how almost every female character has wide hips and a substantial chest with a small waist. They draw them like that because it's sexy, so I guess that makes it sexualization
With dudes like Mr. Gar its not that they're being sexualized, but dudes like drawing buff dudes because it's a "power fantasy" where they admire buff dudes. So it's not looking at dude's bodies as something to be attracted to and desire in a sexual, but admiring their strong physique I guess, as it reflects power.
I personally don't have a problem with the way the women are drawn in the show, I understand it from a design standpoint because it looks good and I think there's nothing wrong with them dressing how they do because I think it promotes being comfortable with your body while tumblr has somehow turned into a puritan colony where female characters can't show skin because it's meant to be sexy to men high school girls should be able to wear whatever they want but a cartoon character can't wear a crop top and shorts, even though I went to school in Southern California and thats pretty much how a good portion of the girls at my school dressed. But at the same time I get how having every woman look the same is uninteresting because from a design standpoint good art should have a balance of variety, and also it kinda sucks how in cartoons when it comes to the designs of male characters they can be as outlandish or as goofy as they want, but female characters always have to be conventionally feminine like in Swat Kats how the male characters are straight up cats but the female characters are just humans with cat ears and pink noses. Same with in Inside Out how the male emotions are a stick and a block but the female ones are basically just humans.