Hiya Viksmeister,
What you are experiencing with your mum is not uncommon in dementia sufferers. Very often the dementia affects their tastebuds too and food which they have probably eaten all their lives becomes unsavoury for them and they start rejecting it. As you are finding with your mum, a sweet tooth seems to be something which many people develop.
As to how to deal with the issue, based on my experience I would mention the following to you:
1. Do not make an issue out of her rejecting food or preferring sweet foods. There is a risk that any kind of confrontation about the issue could lead to paranoia and them refusing food altogether.
2. As you will see on here already, when food starts to become an issue, the best approach (unless there are medical reasons not to) is to let them have what they want. It is better that they eat than they don't eat and even if they add a few pounds in weight during this phase, it can often be a blessing at a later stage where weight loss becomes an issue, perhaps following such things as UTI's etc.
3. There are lots of nutritious ways that your mum can be given sweet food. Things like custards and home made rice puddings, tapioca, things like that can be beneficial - one because you can either load the calories by use of cream or eggs etc or you can restrict them by use of low fat milk etc, but not only that, as the memory starts to deteriorate and the residual memory is often of days gone by, these foods can be those that the person can identify with because they were given them as a child perhaps.
4. Some people have had success with milky sweet drinks, some even give "sandwiches" of Bread butter and a sprinkling of sugar! Meringue nests were a standby for me with my mother as I could pile some fruit on them with a sprinkling of sugar and she'd be happy as a lord. Don't be afraid to experiment to see what she will or will not tolerate.
5. As you have noticed her rejecting foods it is worthwhile ruling out things like problems with her teeth either her own or if she has dentures. Maybe she would eat savour foods if they were soft foods that require little chewing. I know in my mother's care room that wet food was the food of choice all round - eg foods served with a sauce or a gravy like macaroni cheese/cottage pie kind of things.
6. Another thing you could try if her tastebuds are changing is to offer savoury foods but ones which have more intense flavour - perhaps a curry flavour or chilli con carni. Maybe she is not eating things because they now taste bland rather than disgusting.
As always, this is based on my own experiences but hope that it at least gives you some things to consider.
Fiona