I don't see the problem with Rosalina wearing alternative outfits, even if they are revealing.
While I can understand some people might be sexually repulsed, I also don't think wearing revealing clothes is even a sexual thing depending on the context. Like, if Rosalina was taking a visit to say, Sunshine Beach Galaxy (I think that was the name of the beach level with the Piantas in Galaxy 2), I don't see why she wouldn't wear a bikini, or just something with more skin being shown. It's appropriate for the context, who she is doesn't really factor in since I don't think any woman wants to cover up completely on a hot day.
Similarly, while the biker outfit is definitely enticing and Rosalina completely owns the look with her mature body compared to Peach and Daisy, the biker outfit is ACTUALLY a form of censorship, since, well...you'd be constantly seeing her "cosmos" in her normal outfit, let's just put it that way. :V
While it may let her breasts push out more than in the gown, it's not oversexualisation or an exaggeration, since that's just how she is. Contrary to popular belief, Rosalina IS actually quite busty - her gown is just really concealing, and even then, she hardly looks flat in it:
The fact that most people don't notice her bust in any game, gown or biker outfit, is proof that nothing is done in-game to really draw your attention to it. Same goes for the rest of her body.
Essentially, Rosalina can justifiably be called sexy without question, so it's easy to see why she gets a lot of raunchy fanart. However, none of her animations are made to really draw your attention to anything - she doesn't do anything inherently sexual, and none of her animations are exaggerated to make you look at a specific area of her body (unlike ZSS in Smash, where it seems like every animation draws attention to either her butt or her chest, even though they are just fighting animations).
Basically I'm fine with sexual fanart, because honestly the character is beautiful, but I don't want Nintendo do do anything blatantly sexualising with her, like they do with ZSS in Smash and Metroid: Other M. Or like they do with Peach and Daisy in Mario Strikers.
I only dislike the idea of her wearing things that seem uncharacteristic, or off in relation to her overall aesthetic. While I like the biker outfit a lot, I wish the three girls didn't have the same one but recoloured. At the very least, only Peach should have the heart emblem on the back. Daisy should have a flower motif, and Rosalina should have a Grand Star or a Luma emblem.