I'm not great with conformation, and it's difficult to tell with these pictures, but I have a couple things to say...
I definitely think she could put a bit of weight on. She's by no means too skinny, however she doesn't look perfect weight to me. (Then again, our horses are fat, and the TBs I work with at work are racehorses and all too skiny... so I'm not used to looking at healthy weighted horses haha.) She looks a bit undermuscled too. I'm not sure what type of riding you do, but you can't go wrong with hill work. Walk her up and down hills as much as possible- it will help built up the muscles in her hind end and back. It also looks like her chest/shoulder is quite thick compared to her back end, so muscling up her butt would help even that out.
For her neck, make sure that you muscle the top of her neck, not the bottom. If she's always running around with her head in the air when you ride her, the bottom strengthens and she could end up with an ewe neck. However that doesn't seem overly likely because it looks better than when you got her :)
Overall, she looks like she's put together alright. Her shoulder is a bit steep, but that's very common in TBs. She's very cute