Accelgor is only good because it paralyzes while also allowing you to switch out into a pokemon that locks your opponent. If wobb/Stoutland/etc. didn't exist, accelgor would be pretty bad, and accelgor is a stage 1 that hits for 40 more damage.Having Paralysis lock is always good e.g. Accelgor.
Mismagius doesn't hold a candle to giratina.
Giratina would threaten to 2hko your pokemon, which forces your opponent to play out their deck really quickly or else they lose, but it also locks them out of some cards that would allow them to be faster.
Mismagius only threatens 1 ko every 6 turns, which means there's no pressure for your opponent to play very quickly and the lock doesn't really hurt that badly.