Hell No, Kitty
My girlfriend is a smart and accomplished 33-year-old woman who wears little girl-type clothes. She always looks pretty, but there are times I wish she would dress more like an adult. For instance, last week, we had dinner with my boss, and she wore a pink Hello Kitty T-shirt and pigtails. How does a man ask a woman to sometimes dress a little more sophisticated?
I used to worship Hello Kitty, and then I turned 7. Some women do work the head-to-toe little girl look longer than others, but 33 sounds a little late for it. Maybe your girlfriend has gotten in a style rut and hasn't noticed that she isn't pulling off 22-going-on-12 like she used to. Then again, she might be wearing these clothes because she's aged out of them. (Paging Alanis Morissette, because isn't it ironic?)
Clothes say a lot about a person, but there are times your shirt or skirt or whatever really needs to shut the hell up, like when you're accompanying your boyfriend to dinner with his boss. This would be the time to dress to make him look good (more business casual than monkey bars casual).
Tell your girlfriend that you always love looking at her and that you aren't asking her to change her style entirely, just to dress more sophisticated on occasion, especially occasions for your work. She probably has something passable in her closet, but you might offer to take her shopping for a few new additions to her wardrobe. If she's just forgotten to look up and notice she's 33, a new little black dress might lead her to realize it's time to say Goodbye, Kitty, and start dressing in a way that suggests she got out of college about 10 years ago, and not the stroller.