Mom slumps over to the side of wheelchair and won’t try to sit up straight. How can I help?

Seeing people sitting slumped over in wheelchairs all day is one of my pet peeves. If someone needs to spend the day in one then they need to have something better than a basic model, and if families aren't aware of that then the facility certainly is and should be guiding them on how to go about getting one.😠

An occupational therapist should fit the chair to the person's present (and future) needs - you should be looking for something like one of these

https://www.rehabmart.com/category/tilt_in_space_wheelchairs.htm

There are wedges that can be used to help support her trunk.
As a person declines Trunk Support is one of the things that is lost. Just like babies have no trunk support they need to be "propped" or supported until they can sit on their own.
Also if the wheelchair adjusts reclining the back a bit will help. As the back reclines it will make it more difficult for her to slump to one side or the other. Although a wedge still might be needed. If not a wedge a small pillow would work.
Also reclining the chair makes it a bit more difficult for her to try to get out if the chair if that has been a problem.

It's not that your Mom "won't try"...it's that she can't. My MIL is the same way (thanks to a broken back and not enough rehab back in 2009). There are special wheelchairs with taller backs and adjustable sides that help keep weak people more upright. My MIL has one that was ordered for her by her facility. She's on Medicaid. It's very comfy. Ask the admin if they can recommend any, or ask her doctor.