Okay, this is my first time ever replying to one of these, if I get anything wrong, feel free to correct me. Alright!
Why do you think she is acting snappy? Is it hunger, fear, or normal behavior for a young BP?
-Honestly, she's new so she's probably nervous/scared but also hungry. If she's only eating fuzzies but is big enough to go up a food size, definitely go up. Do you have her on a five-day feeding schedule? What's her set up like? A glass tank might make a little one nervous, but at least she's eating. Good sign, that.
What do you think it means when she is poking her head around the top of the enclosure?
-Poking her head around/being active might mean she's hungry. Or, she's looking for a warm place to hide. Again, what's her set-up like? Does she have a controlled UTH? Does she have hides on both the warm side and the cool?
Is it possible to overfeed a BP, or will she only eat if she's hungry?
-It is entirely possible to overfeed a ball python, they are opportunistic feeders. However, I think you might need to go up to the next size prey. You mentioned she couldn't get the f/t fuzzy in her mouth? She might have just been a little confused. Their mouths can open quite wide. Is the fuzzy the same size as the biggest part of the snake? If so, she should have easily been able to eat it.
I hope something here helped. Good luck with your little one! It always helps to upload pictures, if possible. People will ask for them here when giving advice.