It's important to note that showing skin isn't necessarily sexual or attractive (otherwise nude champions like Zac would be way too much) and being covered up isn't necessarily nonsexual (skin tight out fits and such). What makes a champion attractive is a variety of aspects. A common complaint I hear is that people focus too much on females being uncovered or sexualized and not enough on males. This misses the point of uncovered not equating necessarily to sexualized. Just because Tryndamere/Braum/Pantheon/Varus/Ryze has a bare chest doesn't make him sexualized automatically. How a champion poses, what their kit and story is like, what quotes they say and their personality matter in how sexualized they are. Cultural context too, as a shirtless man in Western society isn't seen as sexual as a shirtless woman.
Being attractive or unattractive is similar. I (as a straight male) don't find Jinx attractive because she's crazy and I don't like homicidal people. Her personality and story makes her unattractive to me. Even if she wasn't flat chested, that wouldn't make her more attractive to me, but she would likely be more sexualized depending on how much focus was put on her chest.
On the flip side, I don't think a few scars, scales, or a wart or two makes someone automatically unattractive. It's certainly okay for a champion to be attractive, but different body shapes, faces and such should be part of it and flaws not ignored. Being conventionally unattractive is the same.
What's important is how they are presented visually and thematically I think. A champion's looks should reflect their character and profession or background. Far too often someone will claim that asking for a more thematically appropriate costume is 'slut shaming' or 'too politically correct.' This is wrong. I don't mind Ahri being sexual in her appearance because it fits her character and role as a temptress, same as Eve. The first Cassiopeia made sense too (not much anymore due to that stupid curse in the tomb). If every female champion in League was a temptress sort and wore appropriate attire, it would be thematically appropriate. Lacking diversity sure, but appropriate. Even on champions like Sona it makes a bit of sense because she's a more social character and performer and her outfit might go with that aspect. Being attractive as a performer can be important, though obviously not necessary.
Other champions, being sexualized and wearing impractical outfits are not so appropriate. It didn't make sense for Sejuani to be going around in a metal bikini on the tundra (she doesn't have to be uncovered to be a badass). It doesn't make sense for Zyra to be just a lady with a few leaves stuck on her and with her feet in the shape of heels. It doesn't make sense that Leona wears heels either. Nami is based on a mermaid, but rather than just being a half human half fish, she's pretty fishy all around...but still has her skin split to have rather human cleavage. Caitlin is a highly competent and practical cop...with a miniskirt. They can be attractive sure, but at some point their appearance loses touch with their character and themes. If Caitlin likes to wear a miniskirt off the battlefield or when not on a case, that's just fine. If Leona likes heels, that's fine too, but not in a battle situation. Context for attire is vital. Impractical outfits in skins are fine as those aren't canonical and are just for fun (mostly, as sometimes they can get a bit too much).
Splash arts are a common offender in that regard too. Spine breaking poses and other sexualized aspects can push a champion away from what they are as a character and into just an action figure or fan service. See Sivir and such.
Champions like Vi I don't really see as sexualized (though her behind in her splash art is a bit exaggerated), but I do find her attractive. She's tough and scarred and that's okay. Same with Katarina (though her splash arts are a bit wonky at times and there was that debacle with her candy cane stripper pole). I like Kalista's look and think it fits her thematically, but I don't find her attractive physically. Leona and Diana are certainly attractive and I'd say even beautiful, but for the most part are practical and in appropriate attire (though Leona's heels...grr...).