Seriously though, how do grades at the end of match work?

Urgot isn't too popular. The bar is low.

You probably warded better and/or took objectives, those help your mastery score quite a bit. Also, playing champions in unorthodox positions I find it's surprisingly easy to get a good grade doing so (maybe because it's rare that people do it, and many that try it don't know what they're doing and do it poorly as a result, lowering the bar).

It's 100% because you won the game in under 20 minutes. That almost never happens, and scores are always considered relative to the length of the game, so you basically break the grading system by winning that quickly.

... Did neither team have a support? That's a bizarre end screen on multiple levels.