As Uppercut says Neymar is quality and an improvement on what they had. He was still one of their very best players last season other than Messi, and this is despite never really having a clear run of games to settle in. Quick check says he only started 19 league games in the season just gone.
That being said you're obviously right that signings in other positions were more pressing concerns. That doesn't mean the Neymar signing was a bad signing if they had the money to buy players other than Neymar but decided not to because they are just ****ing idiots (this seems likely). On the other hand if it was a direct choice between signing Neymar or someone like Hummels then yeah the Neymar signing was bad.