Mom has terrible hunched/slouched posture when using her rollator. Does this mean it's time for a wheelchair?

The rollator should have been adjusted for your mother's height when it was first purchased/delivered. So, yes, adjust it now so she can walk straighter and probably in less discomfort. The aide might be able to use a gait belt so she doesn't have to bend over as well.

I don't know if it's time for a wheelchair, but if your mother can still walk, I'd let her do that as long as possible. Primarily using a wheelchair can increase her sense of disability, of being confined and unable to maintain at least a certain level of independence through walking. And it won't contribute to maintaining muscle strength in her legs, unless she's doing PT. I'd put constant wheelchair reliance off as long as possible, assuming that her orthopedic doctor agrees.

That's one of the issues I've noticed about rollators, as opposed to walkers. Although they're more sturdy, they have less of a "envelope" in which the person can walk, surrounded on 3 sides by support as with a walker.

If I were a rollator designer, I'd create a method by which the handlebars could be slid out backward, to act as an extension to provide more "coverage" around a person's body and avoid leaning over as a person walks. I see that limitation whenever I move my father's rollator around.

Some people should not use a rollator. See if the doctor will OK her for PT to make sure she is using it correctly.

Caregivingstuff,
I see that your mom has dementia on your profile. I'd discuss her walking situation with someone who regularly addresses the needs of dementia patients. Mobility can be a huge issue and I get it why you want to keep her walking as long as possible. However, with my LO, her walking was extremely precarious. The balance so poor, that she was falling and getting fractures frequently. Surprisingly, her going to a wheelchair was a blessing, as most of the falls stopped and she was able to use her feet to propel herself around in the chair. (Foot rests removed.) She actually wears out her shoes this way. She gained much more mobility and was able to get all around the AL by herself. Each case is different, but, I'd keep in mind that teaching her to use things differently or to walk differently, when she has dementia is not likely, since she may not be able to remember what she is told.

Is she hunched over when sitting or not using the device?

I'm not understanding who suggested the adjustment, the shoe salesman? Any changes should be made by someone who is qualified.
Despite the sales pitch and all the bells and whistles of modern rollators many elders have trouble with them, my own mom was able to handle a two wheel walker with more ease when inside, we saved the rollator for trips outdoors.

My mom had the same issue with a rollator. She had it as opposed to a walker because she tried easily and need to sit down to rest frequently. That is when exercise becomes very limited. She also had difficulty with the brakes and what they were for. Though she rarely needed the brakes, occasionally down the handicap ramps that seem to be everywhere in some facilities.

Mom also had progressive osteoporosis which was also a contributor to the slouching.

It is so hard to know what the right choices are. My mom lost the ability to walk after a two week respite stay at a nursing home. Looking back now I wish I would have asked for some PT and at least tried to restore her ability. I don't suppose your doc would prescribe some PT? I know that our life became much more difficult when mom finally gave up walking, it progressed to an inability to stand and transfer and this was a big factor in having to place her in a NH.

I have no plan other than to try to hang on and react to the moment. My mother is a mess who has one goal -- the live forever in her house. Last week I thought she was in her last days and talked about bringing in hospice. She agreed. But it turned out she just didn't want to go to a doctor appointment. After I canceled the appointment she was all well again. Doh. I forget that I'm dealing with a child-like mentality now. It reminded me of when I feigned sickness so I didn't have to go to school.

I don't really have much influence on my mother. She is going to do what she wants. I just make suggestions and provide physical support as best I can.

You said it all; yesterday she asked me if I wanted to be her medical POA - I was shocked - that was 5 minutes before she went in to the doctors office ALONE. I probably should have insisted on going with her, but you know, I really think western medicine is mostly about the money for many doctors and facilities now, so am suspicious about most of what they do and want to do. Decided I'm not the person to TRY (if she would really let me) to help manage her health, so am going to ignore it and just let her Dr. and RN, and SW do it. My mil and fil and mom are all cash cows for western medicine, if you ask me; I'd hate to enable that whole routine....

The handles of the Rollator walker must be tall enough to eliminate the stooped posture of the user. The person using it should not have to bend over so far that the posture is in a permanent hunched position. However, there is a possibility that it is already too late to correct the elder's posture from the ill-fitting-for them Rollator.

Ask the doc to prescribe a few sessions with a physical therapist. They can do wonders in ensuring that LOs do things that they won't do if family requests it.