One must ask ‘concerning’ as compared to what, or who?
Other MLB pitchers taking the mound regularly this year. Pitchers with good stuff do exist and pitch regularly in MLB rotations just not the Cardinals so maybe that is your disconnect with stuff based pitchers.
McGreevy’s ERA is fine right now, but he has good batted ball luck and he has allowed some really hard contact. Based off his batted ball data, his expected ERA is 5.68. That doesn’t account for the Cardinals defense, which despite some beliefs, is really good and ranks as 6th in MLB by OAA. His ERA will run a good amount lower with that defense, but maintaining 2-3 full runs better than his expected over a season in unheard of in baseball history. His FIP is 4.42 so still a run worse than his ERA.
Again I see him more of a 3.75ish ERA based off what I have watched of him, but there is luck in baseball and McGreevy has been the beneficiary quite often. But his luck will not always be so good and he will regress to more of the expected ERA for him.