What should I do about my mother not eating?

Being with her 4 days a week is more than enough. Please don't even consider being there 24/7. While basically being impossible it is not good for you and not necessary for your mom. I'm sorry that she has untreatable cancer and dementia. I imagine that hospice is probably telling you not to worry about her eating habits? I hope that hospice is doing a good job keeping your mom comfortable.

In my humble opinion it is her body deciding. It is not at all unusual for someone with dementia to lose all appetite, and even to have almost an inability to swallow. Appetite loss is common with all cancers, treated and untreated.
Have you contacted palliative care or hospice or asked her MD for consults in those areas?

It feels very strange to see someone not eat or drink. My mom stopped eating and drinking. ZippyZee is correct. It is a normal part of the body shutting down. It is difficult to watch. Sending you peace and wisdom as you manage through this difficult time.

Actually, just wanted to thank you all for your responses 💜

My son has late stage cancer. I have been told to not push food at him and to simply support him in eating when he does want to eat.

Her body isn’t experiencing hunger and that’s okay. We all tend to want to fix things with food. Let her direct her eating, or not eating, and she will be okay. Encourage small sips at a time to drink. I wish you both peace

My mom barely ate a thing, a bite here and there.

You can’t force her to eat. Your mom has lost her appetite and so she has no desire to eat. This is a normal occurrence when a person is near the end.

It's a normal part of the dying process, as the body shut's down, it doesn't process food anymore, and could cause pain if you try to force her to eat. Feed her if she asks, don't force it if she doesn't.