First off, we have to deal with the horrible myth that anyone can be great at anything if they just try hard enough. No matter how angry Phil gets and no matter how Elly bellows, threatens, screams or pleads, Michael simply isn't ever going to be anything other than a very mediocre trumpet player and, well, that isn't a bad thing. After all, people aren't die-cast monstrosities that can be programmed to do any task if the right stimulus is given; even identical twins aren't truly identical in their talents so thinking that a boy is akin to a Ford is ridiculous.
Second, what Elly never seems to have realized what Mike was really enthusiastic about: spending time with his uncle. She couldn't simply let the two of them hang out for its own sake because every single second of Mike's childhood was supposed to be devoted to Preparing For Serious Adult Life So Stop Wasting Time so before she'd allow him access to an outside influence, that influence had to be Constructive and cure him of the notion that he's supposed to enjoy the process of being alive because that just spoils everything. The end result is to have Mike vaguely resent Phil for being just another adult who hates fun and wants to kill it because he forgot to be a kid.