No. Mastery scores are based on personal performance and work as an incentive for players to do their very best in any situation good or bad.
If a team loses even though one of them went 200+ CS. 12+ wards placed, 5+ wards destroyed, 20+ kills, less than 5 deaths, and over a dozen assists only to find themselves robbed of a well deserved S, they'd be even angrier that the game basically turned around and said, 'sorry, you weren't able to carry the feeders into victory, no compensation for you'.
And if someone on a winning team scored what was supposed to be an A+ despite being hard carried by the player who rightfully earned their S+ then it'd be like, "EVERYONE GETS THE BLUE RIBBON!" even though it was the one player who truly deserved it.