I don't disagree with you, but my gut instinct for why they've latched onto the imagery of a big breasted blonde woman is pure transphobia. They're trying to purport women like Sweeney represent a cosmic ideal of womanhood and that anyone trans (man/woman/otherwise) will fail to meet her innate level somehow.
They're trying to circle the wagons on aesthetics I guess. They're getting weird about it, saying their preferences are universally and biologically ingrained.
Or my other suspicion is that a lot of reactionaries are closet bi/gay and are using the abstract ideal of Sydney Sweeney to hide how gay they are? I don't know. We've gotten into weird territory.