I feel like the ending should bother me on principle,

Carmilla's vampirism is brought to light by a traveling character who enters the narrative at the end, functionally, in order to resolve the plot.

But, honestly, I didn't really mind it much. I found the whole book pretty enjoyable. 

As far as "lesbian vampires" go-- I hadn't realized the girls were so young, and Carmilla's more romantic feelings make the protagonist uncomfortable (and, in turn, made me a bit uncomfortable, knowing how unwanted attention feels), so be aware of this.