Do you think she needs to lose more weight?

I think her weight and overall condition are good. she just has a bit of a tum. riding 3 - 4 times a week and you should start to see more of a top line in 4 to 6 weeks.

I think working with a chiro and getting a plan of improvement together with him would be a great start. Her back is very low and has effected how she stands in the hinds, canted way under herself. Her hind feet are trying to hold up her back, which is not doing its job. He would release things that would allow her to build muscle properly and exercises to help develop and re-train the muscles to be able to lift and hold her back better. I suspect she needs work at the withers and at the tallest bump on her rump...the sacro-illiac, but its amazing what they can find that's "gone out". She's not standing squarely, her shoulders are canted to one side. You won't be able to build muscle without adding pain, if you don't free her up, get those muscles straightened up and then developed to stay straight with the correct exercises.

The good thing is that she has a good trim on her feet. Stay on top of that...that's important too. Also pay close attention to saddle fit.

She's a sweet mare...

she looks good weight wise, just out of shape. he gut should get smaller if you ride her 3-4 times a week like you say