If we're describing Mata as a flop then there should be another category several rungs below that for the likes of Fred and Di Maria, as the disparity between the service we got from their respective transfers is so big as to be not reasonably open for any sort of comparison whatsoever.
I mean Fred cost approx £52m pounds, has made 31 appearances, has barely played well in a single game since arriving. Di Maria cost approx £60m, lasted just one season, had only a couple of month of good form before leaving in rancorous enough circumstances that he still attracts the disdain of many United fans. These were real flops.
Mata on the other hand cost approx £37m, has been here for seven seasons, has made 227 appearances for the club, has scored 45 goals, scored a vital goal in an FA cup final, contributed goals and performances in many big games and has become very popular with United fans. He might have been a disappointment, sure, but an outright flop? Nah. He's been here too long, been too good a servant and won over the affections of the fans too thoroughly for that term to qualify imo.