Reminds me of amateur table tennis. There is a strange dynamic where you down regulate your performance unconsciously when the opponent is playing worse and vice versa.

Could be described as some physical form of this effect: https://en.wikipedia.org/wiki/Asch_conformity_experiments

The term would be conformity / normative social influence.

Surely this has nothing to do with specifically table tennis, nor that it's amateur.

A better example would be mixed boys/girls sports classes in school, where the boys deliberately hold back as to not injure/scare the girls.

It's a pretty obvious and human thing not to go out and completely destroy a much weaker opponent. We're social animals after all.

There also may be an element of energy conservation, there's objectively no need to put in any more effort than necessary. Inefficient.