Most Helpful Girls
as a mother I think its very sweet but you have to also think about possible complications:
is there anything in your note that could upset your girlfriend?
does she encourage you to interact with her mom?
is her mother controlling in any way and would be encouraged to become involved in your relationship?
are you just trying to impress her mom?
al least write a letter on nice stationary or buy her a card and write in it.
don't' just hand her a piece of note book paper
personally I think it's a little weird if you've only seen her a few times and don't truly know her that well. but if you feel like you know her well enough, then go for it.