>>12680200A smogonfag is a catch-all term for people who are into competitive Pokemon battling. Namesake comes from the site Smogon, which is one of the largest sites for the competitive community.
A Karenfag is considered the opposite, but isn't necessarily so. Basically, a Karenfag is a now warped catch-all for people who are against competitive battling. Originally it referred to those who play and try to win with their favorites, based on Karen from the games saying something along the lines of "Truly strong trainers try to win with their favorites"...
Of course, that got warped after awhile like most shit does and is now sorta kinda considered Smogonfag's worst enemies.