Is Neeko not good in high elo?

Of course she is good in High Elo, as good as any Lux/Morgana/Zyra type of kit (straight line snare + area damage). The key is using her effectively with her kit, which makes an effective roaming tool and other wise tricking the enemy to engage on the wrong person, ending up with them getting caught in the straight line snare/area damage combo for high burst as the ally team descends upon them.

Neeko is Neeko, and will be nerfed to oblivion for most of S9. Why bother to master her before they find her spot?

Less effective sure but she's not bad

The W is a deceiver ability which is pretty useless against players who know what they're doing - The ultimate is busted strong, one flash R and the enemy team is stunned or dead lol