My mom uses diapers, but she cannot hold her weight up for me to pull up or down her pull-ups and/or pants. What do you suggest?

I had a client who LOVED me b/c, in her words "You are so STRONG!" Well, I used to be--I was killing my back, moving her, as she would go completely boneless at times and have me tugging and hauling at her to get her to sit upright so I didn't have head in my lap as I was driving.

One day, after PT, the PT walked us out to the car. I asked him if he could help me with some maneuvers that would help her help ME. He had some simple moves where she was doing the lion's share of work and I was there to control the moves. I have long since forgotten, but a person will take all the strength out of you if you give them half a chance.

Get a PT to help you with these moves. Both getting her in and out of the car became much less onerous!

I'm sure, had I needed to, the PT could also have shown me ways to change her depends. By the time she needed those, the family moved her to a full time living arrangement. But I know if I had needed it, we could have had some kind of training in helping her.

Sounds like it is time to put her in a nursing home.

Get the brief diapers with tabs. While she is laying in bed, roll her on one side. Slide the brief under her, roll her over on top of the diaper. Then, roll her over to the other side. Position the diaper so it will cover her bottom. Roll her back upright and affix tabs. When it’s time to change her, it will be easier to clean her up while she’s laying in the bed instead of trying to get her up to the potty. This is my life. My mother had a stroke and even through pt and continued exercise, has no strength to stand without help.

This brings back unhappy memories, I'm afraid I spent a lot of time shrieking at my mother to "stand, stand, stand the eff up" as her legs gave out and she'd slowly sink to the floor.
How exactly are you getting her to the bathroom and onto the toilet? If she's in a wheelchair how are you doing transfers? How are you dressing her? What about bathing??

To answer your question - you can tear the pull ups down the side to help in removing them, or you could try tab style briefs. If she is in a wheelchair you could buy (or modify) pants designed for wheelchair users that have an open back (like these)
https://www.buckandbuck.com/womens-fleece-back-flap-pants.html

Depending on her ability and if she is willing you could help her to work on some simple PT exercises to help strengthen her legs, these
https://eldergym.com/leg-exercises/
are identical to the ones my mom's PT had her doing.

If she had something to hold on, like arms, could she hold herself up long enough? You can put a commode over the toilet. New ones come with a bucket and splash shield. You will use the shield. There is a bar on the back if the commode that u remove. Slip the commode over the toilet. Legs can be adjusted but not too high because tge shield needs to go in the toilet passed the rim. The commode comes with its own toilet seat so you can remove the one on the toilet or put it up. This gives the person some stablity and arms to help hold her up.

Have you looked into getting a Sit To Stand?
This is a great piece of equipment that will help your mom stand, she does need to bear some weight. You can use the Sit To Stand to transfer her from a chair to the toilet and back or in and out of bed.
If she can not bear weight at all then the piece of equipment that you would need would be a Hoyer Lift.
Switch to a Tab Type brief. She can be changed laying down. Or if she can bear weight for a bit the tab briefs can be put on while she is standing.