I saw the incident, didn't see what was wrong with it. I'm guessing it's only because it's Raheem Sterling that was showboating, but this is honestly something that shouldn't deserve any spotlight whatsoever. Mata was only frustrated because of the time wasted, that's all.
The level of acceptance, in my opinion, would be dependant on when the showboating happens during the game. If it's done during injury time, that's bad manners to me. But then again, who doesn't do this? What happens the most is the scoreboard. If Sterling had done this in a game where City are behind, then that's a different story. For as long it raises their chances of winning the game, they can do whatever the hell they want.
I don't really understand the deep culture in la liga and spain, but it seems like the players and the people as majority doesn't approve showboating. it's funny how sterling and mata's incident (manchester derby) has brought me to open this issue, and the fact that the culture has been going on from years. probably influenced from south america, neymar did it many times, notably against bilbao, where there are much confrontations. However, ronaldinho did it too but he didn't receive the similar reaction. so what's the level of acceptance? what's the point of football match? is it not for entertainment?