Mom is suddenly angry and aggressive. Do dementia symptoms accelerate in the final stages?

Nobody here can tell you, with any certainty, that this is 'not a UTI'. Only a doctor can tell you that, and after s/he he takes a urine culture, which you may want to pursue.

The symptoms of dementia tend to change all the time; sometimes rapidly and sometimes very slowly. My mother with dementia would go along at a certain pace for a long period of time, and then suddenly, she'd have a rapid decline or take a 'step down', and remain at that level for a while. Until she took another 'step down' in her decline. As her dementia became advanced, so did her agitation and anxiety; your mother exhibiting signs of anger is a sign of agitation. What worked well for my mother was Ativan at .25 mg of a dose to start. Hospice had to up her dose to .5 mgs as her agitation worsened with her decline as she approached the end of her life. And yes, she got worse VERY quickly as the end of her life approached.

Have you gotten a hospice evaluation for your mom yet? You can ask her doctor to write an order for one, then call the hospice company of your choice for an assessment. My mother's company used "Senile Degeneration of the Brain" as the Medicare approved reason for their service. They were wonderful in helping her stay comfortable and anxiety free during the final 2 months of her life.

Babbling and using senseless words with dementia is known as aphasia, and is also common as the condition progresses. My mother was unable to understand words as her dementia became advanced. She'd keep saying WHAT? when I spoke to her and at first I thought she couldn't hear me. But then I realized she could hear what I was saying, she just couldn't comprehend the words! It's a sad thing to witness, I know.

Wishing you the best of luck with a difficult situation.

This incident seems to be the result of her advanced dementia. This is not UTI in which the bacteria and toxins cause an acute delirium. If she didn't have dementia, yes the first though would be delirium, In a patient with advance dementia, the first thought should be dementia, unless proven otherwise. Your mother will require placement in a more secure environment.

Sudden changes in behavior are always indication that doctor needs contacting. Take a clean urine specimen with you, as UTI can cause these changes.
If there is self harming and out of control behavior there may need to be emergency transport to the ER and admission for assessment.
You may be living now with end stage behavior that is going to require memory care so that it doesn't soak up every waking moment of your life.
I am sorry you're going through this and wish you luck.

Sudden changes in behavior are sometimes a result of a UTI. Have her tested to rule out. If your mom is in end stages, she should, be able to qualify for hospice. Either way, I would have her tested for a UTI today.