We never book a mini-suite on Princess. Although DW does not use a wheelchair, she has mobility problems with her knees. Except for accessible cabins, Princess mini-suites have shower in tub, and that would not be useable for her. We will book a full suite, a balcony cabin, or an outside cabin - but not a mini-suite. I would recommend you not book a non-accessible Princess mini-suite. If you book a balcony or outside cabin, make sure you mark it "No Upgrade" so they don't upgrade you to a mini-suite.
You stated she does not need the wheelchair in the cabin, so it is only an issue of storing the wheelchair and getting it through the door. There would be room to store a wheelchair in a full suite. In a balcony or outside cabin, there would be room to store a collapsible wheelchair - but possibly not one that is not collapsible.
Would you be with your mother to help her every time she enters or leaves the cabin? If so, a collapsible wheelchair could work even if it were too wide for the door. She may need to stand up outside the cabin and use a cane to walk through the door while you collapse the wheelchair to take it through the door - and do the reverse when leaving.
Another problem would be the cleaning carts used by the cabin stewards and left in the hallways. It may not be possible to pass by the cart to get through the hallway. If the cabin steward is there with the cart, he would move it out of the way, but the carts are often left unattended. To get by, you may need to move the cart for her. Another option would be for her to stand up and walk past the cart using a cane and for you to collapse the wheelchair and move it past the cart.
On one cruise (pre-COVID), DW was using a collapsible walker for walking long distances. It would not fit through the cabin door without collapsing, and it also needed to be collapsed to get by carts in the hallway. However, we had no trouble using it.