My mom has dementia for last 2 years. Is it usual for them to talk to themselves?

She is. Thats why I never answered my Mom. She was just speaking what was going thru her mind.

I do it all the time! I'm the only person who listens to me!!

It's not unusual at all. Here is a thread on that very topic back to 2016:

https://www.agingcare.com/questions/mom-with-alzheimers-muttering-talking-to-herself-209750.htm

Hope it helps!