I mean as far as she knows, the robed guy is presumably human and she was able to easily 'overpower' Will in the human body , idk if slime possession strengthens you (not that Lady Jelly was weak, given the taste for torturing ppl), but it is interesting in contrast to the "weakest demon" that the summary describes slimes as, and 'easily defeated by human adventurers' in slime form or so, because i imagine that most slimes wouldn't go out and be confrontational with other creatures
I'm honestly less and less sure how "lowly" a slime she is, given everything she has shown to be capable of, and considering that her mentor apparently survived a head-on collision with a hero.
Though it'd be admittedly really funny if slimes in general just are like that in this universe