I've worked for a lot of maintenance departments. I've yet to meet a manager who wasn't a mechanic. Ever.
Most are experienced mechanics and inspectors.
I've been a working A&P for a lot of years now, and have yet to see any standard in the FAR dealing with a requirement to repair lawnmowers or cars.
There's no federal certification for lawnmower mechanics nor for automotive mechanics, but there certainly is for those working on aircraft.
There's not a damn thing in AC43.13 about repairing motorcycles.