We have gun wielding outlaws, a literal deal with the devil type demon in Tahm Kench, a Yordle with a blowdart, a giant growing Void Monster, whatever Jax is, Bard and Ivern. How does Zoe not fit into that theme, as a bubble wielding mage/Aspect of Twilight? She's just as fantasy as everyone else is in League.
Kled still looks pretty silly compared to likes of Trundle, Lissandra, Leblanc or Azir, he's a hairy, angry, crazy Yordle riding an immortal lizard. Sure he looks better now thanks to Riot Artists evolving their art styles, but so does every other new champion, including Zoe. Just compare her with Annie, another childish champion and see which one looks better.
What do you mean by design, artistic, gameplay or both? Lulu seems perfectly fine by all standards, considering she's been pretty popular in the meta and people like playing her (and how she looks). Teemo is outdated, but he's still functional (and looks alright) compared to other champs slated for reworks/updates like Irelia, Swain, Aatrox, Kayle and Morgana. The angel duo look much worse than Zoe in game (which is what we've seen so far) and there are more just as bad.
People are just overreacting because Zoe looks different and brings new mechanics, that's all. That's hardly different from the likes of Kayn, Ivern, Camille, Xayah/Rakan or Ornn, all of whom look vastly different from one another and bring new/updated mechanics to the game. And it's not like they were all received well, Camille was hated due to being insanely broken, people hated Xayah's attitude as well as Kayn's, while they found Rakan either funny or obnoxious.