I think it is okay to go by Lizzie as an adult, it's pretty but note overly cutesy. I think it sounds much prettier than just Liz. I've heard of adult Elizabeths who go by Libby or Bessie so I don't see why Lizzie would be a problem.
I knew a Liz who, at 21, decided that she wanted to be called Lizzie, and that was that! So you could say she went the opposite way. I also know another professional Lizzie. At first I was surprised that she didn't go by Liz - every 20-something Elizabeth seems to want to go by Liz - but I find it really refreshing now. Actress Lizzy Caplan - she played Janis in Mean Girls and is gorgeous - is a good example of an adult Lizzie/y.
Oh and I was talking names with yet another Liz and she was saying how, when she was younger, she went by Libby, and she wished it had stuck, preferring it to Liz. I thought that was interesting, since Liz is often cited as the more "grown-up" nickname for Elizabeth yet she clearly liked Libby more.