I remember a friend coming back from her first term at uni having put on a LOT of weight - she looked shockingly different, but I didn't mention it because I'm sure she knew! It was just adjusting to living more independently, eating what/when she wanted and drinking with friends. She went back to normal when she'd settled down.
Oh an as for hair and whatever - she's young, she's experimenting, meeting different people and being influenced in new ways - that's what going to university is all about. We all have shit haircuts to look back on... it's not a big deal.
But about why you feel that way OP... I'm not sure, but at least you are questioning it and trying to have a look at yourself. Did/does your own mum (or anyone else) put that kind of pressure on you, do you think you've absorbed it somehow? Also, there are a lot of mixed messages out there - body positivity and not wanting to fat-shame, which I do support, but also the message that weight is a terrible health problem, children being weighed, the idea that we are crap parents if we have an overweight child. Which there is something to as well - health is important, but you don't have a right to give someone a hard time about their body, and that includes your own DD. It's a hard balance to strike.
I would try to take a step back, stop thinking about bodies and think about what your DD needs most when she's at uni - a loving, safe home to come back to and a mum who has her back 100%. Make her her favourite food, go shopping with her, or to the cinema, or whatever you like doing together. Think about what she needs and wants from you aside from appearances. Forget the gym, salad etc. - those are her choices to make and not yours, and pressure won't help. Tell her you love her and ask her about how she is and how it's going, and if she talks, just listen.