Kes is amazing. She's less than two years old when we meet her, but from the beginning she projects the wisdom, serenity, and self-possession of a Vulcan elder, but with deep compassion as well. She was victimized, but didn't let it break her spirit or dull her joy at discovering new things, her willingness to reach out to people and inspire them.
I wouldn't say she was "sweet," not in a Disney-saccharine way. She was kind and caring and gentle, but with an underlying poise and dignity that I found very impressive.
Granted, the writers largely wasted her character by keeping her stuck in sickbay; with her short lifespan, she should've been devouring life, eager to expand her horizons and discover everything while she could. We did see some of that in season 3, though. And she had a great rapport with Janeway, really bringing out the captain's maternal side.
And "wooden"? I can't understand that reaction. Jennifer Lien did a superb job, controlled but sincere. She did a great job when given a chance to stretch; her performance as Tieran in "Warlord" was nicely menacing and an impressive departure from her usual performance. I can't remember ever being disappointed in her acting. And she had a really beautiful voice that I just loved listening to. (She had a brief career as a voice actress after Voyager
. You should hear her as Agent L in Men in Black: The Series
-- an incredibly sultry and sexy vocal performance.)
And when they finally let her ditch the pixie wig and use her real hair in season 3, she looked magnificent. Particularly in that black catsuit with the velvety fabric.