My mum has dementia. When she started to decline I gradually took over 'doing' Xmas for her. If she is in early dementia, she probably gets stressed or nervous easily and the fear of forgetfulness can make someone 'hate' things they used to enjoy. The key is to reduce what she does and start to transition her into a 'guest'.
Shopping - At first I took her shopping and she would select things and wrap them. The next step was taking her shopping and my pretty much saying "Oh X would like this" and putting it in the basket and my wrapping whilst she watched. Next step was my just doing her shopping for her and bringing it over for her 'to see' then taking them to my home to be wrapped and returned.
Decorating - This went from me doing her 'full on' decorating with her telling me what to do to gradually reducing the number of decorations to her little artificial tree, a few Xmas throw pillows, and small nativity. It ended with me doing the decorating while she watched.
Food- My brother lived with her and always did Xmas dinner so it was just a matter of my taking over a few sides and telling Mum to just 'relax'. We learnt that it was best if she sat away from the kitchen and if we were as quiet as possible (avoid crashing pots and pans, set the table without clattering).
Mum went to assisted care 3 years ago. The first year she came to our house, stayed a few hours, and enjoyed it. Second year she came but became fretful and wanted to leave right after dinner. Last year we made the decision to eat with her at the care home. She didn't really 'take in' that it was Xmas but the facility does a nice lunch so we ate with her and then went home and had our own Xmas dinner later. It was really hard.
Try to make it clear to your mum that Xmas is on you now and she won't need to lift a finger. If your mum lives next door, let her come and go as she pleases. Try to get her to come for dinner but if she says she doesn't want to come, don't force her, take her a plate. But do send someone over a few times to see if she's changed her mind.
Dementia is a true bitch.