Double standard is part of it, but sexualization isn't just about how much skin is showing. This outfit shows about as much skin as the Conan cosplay, but is clearly more sexualized:

A Supergirl or Powergirl cosplay can also be revealing, and while you might find someone in that cosplay (or the Conan cosplay) sexual attractive, they're not as sexually suggestive as what this girl is wearing (which is essentially some belts held together with body tape, I presume). The line is blurry and dependent on context, but I would say this is over that line.

Neither is unacceptable depending on context.

Walking in a supermarket? Both unacceptable

Comic-con? Both are looking for attention. But no touching, you weirdos.

It appears that the NYC Comic Con guideline is "Skimpy costumes are permitted though concessions to accuracy are needed if skimpy to the point of negligible coverage i.e. less than typical swimwear."

It is, of course, up to the enforcer, but I would evaluate the Nonon cosplayer as "less than typical swimwear", and the Conan cosplayer to be about the same as typical swimwear. Some skin-colored underlayer would've made it more safe.

I would feel uncomfortable, if that dude in his shorts approached me.

Did anyone say it was unacceptable? You think dollar store Conan here isnt being touched just as much as the girl in the OP?