Tremors and stiffness due to Parkinson's, MS or eneral instability or do make certain garments a challenge to put on. Buttons and snaps are difficult to use. Therefore simple over the head styles or easy to step into clothes should be considered.
Also, easy to step into clothing that can be put on while seated instead of having to stand up, will certainly help reduce the risk of a fall due loss of balance.
Dresses with large zipper pulls, 'muu muus' and pullover head style tops, or pant sets can be helpful.