Is my ball python underweight??

Does my 6 month old ball python look underweight? The breeder I got her from says she’s about 75gr. Which I’ve come to learn she should be over 120.
Heres a couple pictures of her:

http://imgur.com/uYzfp2p
http://imgur.com/OLWkWlo

I just got her two days ago, so if she is underweight, when can I offer her food? She’s adjusted well and has been using her hides when she’s not looking for food. Her head wobbles a little when she tries to climb... she looks weak.

Nope, she looks good.

To me she looks like she's a little bit thin, but it doesn't look like it's at a dangerous level. She stills looks healthy. Typically you should give a ball python at least 7 days to settle in before feeding. Once she starts eating she should gain a little weight. You should probably start weighing her about once a week to make sure she starts gaining weight. I wouldn't worry about her too much unless it seems like she begins to lose weight or look unhealthy.
She's a gorgeous snake and you obviously care for her! I hope you have a wonderful experience with your new pet. Good luck!

For 75g at 6 months old (especially for a female) I was honestly expecting her to look worse, but based on your photos her body shape actually looks pretty good. Rather than being underweight, she might just be a baby who started out on the smaller side straight out of the egg.

To answer your question then, give her one full week to settle in (so 5 days from now, if you got her 2 days ago) before you try feeding her. Feed her one prey item that is appropriately sized for her (I'd guesstimate large hopper or small adult if you're feeding mice, or small to medium sized fuzzy if you're using rats), and space out feedings every 5-7 days.

I wouldn't say the head wobbling you're describing is necessarily due to weakness. The more notorious "wobble" is the neurological issue that is associated with some genes like Spider and Champagne, and can vary quite a bit in severity, but often does not affect the snake's ability to feed/thrive in the least. However, you'll also see a slight unsteadiness in the head movements of any healthy baby ball python, just as part of the motion of breathing and of their tentative movements as they're exploring. Also note that young ball pythons have proportionally larger heads and thinner necks compared to their adult counterparts, so moving those cute noggins around takes a little extra effort on their part!

And speaking of cute noggins, your girl is adorable! Congrats!

She looks fine for her age - she will bulk up a bit when she is older but she difinitely isn't underweight or anything. :)