Klefki is more of a support type pokemon, I think. Sweepers are split into three categories: Physical, Special, and Mixed.
Physical sweepers are pokemon where you usually invest all EVs into Attack and HP or Speed or something, while you invest EVs into Sp Atk and HP/Speed for special sweepers. Mixed sweepers are pokemon that usually have good base stats in both Attack & Sp. Attack, so you would invest EVs into both of those stats to boost it. Those pokemon are usually all offensive and have all offensive moves, with the exception of stat boosters or protect. Some examples of sweepers are like Mega Kang, Aegislash, Greninja, Mega Scizor, Mega Mawile, etc. Pretty sure all mega pokemon are sweepers, but I might be wrong. Physical sweepers also use physical moves (which is identified by the little picture in the rectangle for a move), and special sweepers usually only have special attacking moves. Mixed sweepers can have a combination.
There are different terms that apply to competitive pokemon, including sweeper, like: support, special/physical wall, & staller are ones that I can think of off the top of my head. This link may help you with some of the roles/terms for them :
click !
I hope my explanation helped, and that it made sense. This is information I've gathered from my experience in competitive battles. c: