Can I fly a model aircraft or UAS over a stadium or at airshows or sporting events for hobby or recreation?

No. FAA guidance says that model aircraft flights should be flown a sufficient distance from populated areas.

Air Shows and Sporting Events
For aircraft operations in the vicinity of aerial demonstrations and major sporting events, 14 CFR 91.145 gives the FAA authority to establish Temporary Flight Restrictions (TFRs) to protect persons or property on the ground or in the air, to maintain air safety and efficiency, or to prevent the unsafe congestion of aircraft in the vicinity of an aerial demonstration or sporting event. In practice, TFRs issued under 14 CFR 91.145 are issued primarily for air shows. The FAA determines when a 14 CFR 91.145 TFR should be issued for a sporting event on a case-by-case basis.