There was nothing to suggest that Diantha was holding back. Nothing at all.
As for the writers not letting him win? That I'll agree with. But if you're going to factor that into your argument, then you can't really say that XY Ash is a very good trainer at all, because when he would win in any other circumstance, he loses. It doesn't matter when he wins or loses because he will ALWAYS lose in the end where it counts the most.
You could argue on that basis that if it were Paul in the Kalos League finals against Ash instead of Alain, Paul would win. So really, the question of XY Ash vs. Paul is entirely situational: how many times has he won and lost prior to that, and what are the circumstances? If he's lost once before, he might win this time. If it's a league match, he'll probably win. League finals? He'll lose.