The rules state that

"Mission Points: The game ends when, after the resolution of any action, a player has achieved 20 or more MISSION POINTS (from any source)"

It does not say "any single source" so am i understanding it correctly that if you had multiple characters with abilities that accrued mission points they are basically pooled together and trigger when you have 20 total?

Example - Im playing Uatu and Ares on the same team. Uatu has gained 8 missions points through his trait and Ares has gained 12 through his trait, would the game end with me winning?

yes you win.