I feel like it wasn’t just Kes who had this problem.
The Doctor and Seven were probably the best written characters. Tom and B’Lenna were probably the next two after that. Janeway only got better because she could act as a bad parent to Seven, which vastly improved her character and gave her focus. Neelix and Tuvok kind of drifted off to the background. Kim and Chakotay were blander than that, although Kim got a few decent character beats.
I’m not going to fault the actors on this, since this was the writing.