In the book Humbert makes mention of her monkey-ish feet. Lolita was a lanky nymphet and I think showing her feet in the movie just illistrates that.

Good question. Nabokov wrote a lot of lines in the novel pertaining to feet. Lyne tried pretty hard to stick close to the novel. I know I read either in Keys to Lolita or the Annotated Lolita what significance the feet meant but I can't for the life of me remember exactly what.
If you haven't seen the 1962 Kubric version the intro to the movie is comprised of nothing but young girl's feet having her toenails painted by an older man's hand. It's brilliantly done and actually seems to sum up many of the questions that arises from the story. Is the girl being forced to comply to the older man's control and domination,or is it the other way around- the older man subject to pleasing the young girl he is obsessed with; quite literally "at her feet?"

Bare feet can be associated with youthfulness.