I think it is, since a slime is like music. You can't cut her, shoot her, hit her or hurt her, but she sure as hell can hurt you. I can't see someone who is literally untouchable having any other kind of personality. that said, I also understand your view. A slime could also be like water, being more of a "Go with the flow" freeloader, with no strong opinions or personality. But put in that context, which makes the more interesting character? besides, the MC sort of fills the "Freeloader" slot already... but in the end, I think the creator wants you to look past their bodies, their features, and just see his characters as "People" We've got an elf who isn't all sunshine and rainbows or particularly bright (Pun Intended) a Cat-girl who isn't a seductress (Gasp, she's actually a retro-nerd! *Thank you!!!*) and a spider who is anything but cold or bloodthirsty, but rather girly and sweet.
It's honestly pretty damn admirable.