Well, the money doesn't really make much difference here though. If you look past the numbers, his performances would still be disappointing had he been 20m based on his reputation alone - even if he came the year before I think. Last season was easily his most consistent, and the ones before might have fluctuated alot but not to the extent of this season. Infact, you could argue he's been fairly consistent. Really good for a while, then poor. Burnley he was decent but besides that it's been either a fairly gradual decline, or simply a ton of mostly equally poor games but with the faith and leniency offered gradually dwindling.
The whole 'managers fault' bit is fair. But I don't subscribe to that thinking either. Sometimes players and systems just don't fit, and won't fit. I get that it's a 'managers job to get the best out of players' bla bla but ultimately it's their job to get the best out of the collective. And unfortunately we've done better results wise since becoming more controlled than at the start of the season where Di Maria looked great.
I don't think it's a permanent issue with the club or the league either, past the usual new surroundings and settling in lark. I'm confident that in the future we'll see some more expansive football that Di Maria can shine in. He'll still have some shite games, 'cause that's the type of player he is. But that'll be because flair is generally inconsistent and frustrating, it doesn't always pay off rather than a player who looks somewhat lost and isn't playing on instinct. He's thinking too much, trying too hard, it's clear to see.
As for the league, @jojojo made a point earlier that his early games render that point moot (I think?). However, settling into the league is as much about the long term rigorous nature than some honeymoon period of fixtures. It's not going to help things, but it won't last..
Ultimately there's a combination of mitigating factors, which definitely serve up as a good excuse. Whether they're the true cause or whether Di Maria can bypass them it's too early to tell either way.