OP, it sounds like your mum is receiving the appropriate care.
You certainly can deteriorate in all kinds of ways if you have brain mets, which if she has three types of cancer, or rather cancer in three places this may well have happened, it does render you unable to walk and talk, or she may have had a small stroke. Once the body stops working effectively in one way, other ways also start to go wrong, and the overall effect is just catastrophic for the body. You end up with infections, strokes, heart problems and the cancer itself all causing multiple problems. You can't just remove say the speech one and make everything better.
If she is already on an end of life pathway, these days they let the patients usually dictate if they want drinks or food, as many people lose their appetite at this stage, and also swallowing is problematic (hence the use of the thickener).
The body is adapted to die though, just as it is adapted to be born. I find Hospice Nurse Julie on Facebook and Instagram's videos very helpful to explain the processes that go on during dying, there is a pre-dying phase and then an active dying phase and it helps to know what types of things might happen.
Treating your mum actively with antibiotics might help her live temporarily longer but it might be more drawn out, so that's something to bear in mind.
Moistening the lips is also used to stop people feeling uncomfortable if they are not drinking.
It may be your mum is not there yet, and she may still want a little food or drink, but ultimately nothing is going to make a huge difference and giving this may cause her to choke towards the end.
I'm sorry, OP, this is such a hard thing, talk with the nurses and people around her, phone Macmillan or another healthline as well. It is a hard time, your mum will also find it hard, I find it helpful to say 'I don't know, let's ask the doctors/nurses/chaplain' in these situations when asked questions I don't know the answer to.
Wishing you all the best at this time.