>Start playing multiplayer game
>Do really well
>Start questioning of you're actually good or if the other players are just trash
Usually it's both.
These days pretty much every game will have an ELO system, either visible or behind the scenes that will eventually move you to a level where you win half the time, but usually they'll start you off playing with people right in the middle of the spectrum. If you're winning a lot early that means you're above average, probably