Parasite is mostly decorative without any reliable use.
Visible damage of parasite level 3 can be seen on mobs about level 50, later mobs won´t be very impressed.
Aside from having fun watching those little boom crawlers running around in weird directions, parasite explosions do count as attackskill with the known effects concerning level 100 zombies.
Also, parasite explosion damage doesn´t seem to stack up, so if two explode on an mob, only the first explosion - if any - will count.
Even succeeding in luring a mob on a "sleeping" parasite will not always result in an explosion and only few explosions will do damage.
Still fun to have it.
btw: please be careful in personal dungeons, there seem to be some weird however also strictly decorative consequences when parasites turn to enemies in personal dungeon number 4.
You might have 240 of 210 available mobs on your killcounter, if the bug happens too often there and you have to kill your parasites - without any real consequences for your rating though.