It's absolutely for the best he stays up, sure you could keep him in St. Paul to build him up for longer outings if you want to use him as a starter in case of future injuries or a possible trade at the deadline, but some of the metrics in St. Paul obviously didn't carry over to the majors yet. His Sweeper had an insane 45% whiff rate in St. Paul with a 41% chase rate while so far has only been able to get 23% whiff in the majors with a 34% chase rate. His other pitches have actually managed to tick up in whiff rate in the majors which is an interesting development, but the almost half drop with his sweeper has led to his overall whiff rate dropping.
Having him in St. Paul I don't think will improve how he works his pitches in the majors, letting him face major league level talent and working on the right way to use his stuff to fool batters is the only real path to let him develop properly. I don't think his current issue is a stuff issue, it's a matter of optimization at this point and sending him down to face AAA batters won't improve that.