I don't know if this is an accepted definition, but the best description of "a scrub" I read was "somebody who has a preconception of how the game ought to be played, and moans and bitches when other people think differently". So, a scrub isn't necessarily a noob, or a tryhard (but could be both).
So, if you're playing a fighting game with somebody who stomps off because you got a "cheap" win through corner-trapping, or scoring a ring-out, or using timer spam, or throwing "too many" fireballs... they're a scrub. They entered the match with a bunch of house-rules in their head and got mad when you didn't read their mind and abide by all these arbitrary limitations that (funnily enough) tend to favour his play-style and disadvantage yours.
Screw that - the rules of the game are the ones the game came programmed with; and the acceptable ways of getting that win condition are literally anything the game permits you to do (short of knocking your opponents controller out of their hand).
So in the above situation, is OP a scrub? If you just play low-tier characters because you genuinely prefer them or like a challenge, no. If you avoid high-tier characters because of some moralistic idea about them being overpowered or "just for tryhards", then possibly that's a bit scrubby.