Of course he could score more, same goes for Martial and pretty much everyone else. Players miss chances.
You get more high quality chances though if Bruno Fernandes, Pogba, Zlatan, Rooney etc. provide you with nasty through balls putting you one and one against the keeper rather than some Dan...
Obviously not "crap", just not worth what he would cost.
Scoring 56 goals is good but it is easier to score in United than in for example Brighton. If Maupay plays for United, where he'd constantly get world class service, for seven seasons I could easily see him score more than that.