My mother has dementia. When she sits she rubs the top of her mouth with her fingers backwards and forward. Is this a tic?

Sounds strange but could she have had a hot beverage that burned the top of her mouth?
You know how the skin shrivels when it has been burned? Maybe she's trying to get the loose skin off (?)

However it probably is the dementia.

Have you asked her if anything is bothering her? Maybe a quick trip to the doctor might help.

She has been to the dentist n yes she had some bad teeth but I think it is the anxiety, when she does this she looks like she is thinking n sometimes she just ask all kind of questions that she asked before over n over and then sometimes she doesn't say anything

People with dementia often do things the rest of us would consider rather odd, from carrying around stuffed animals or dolls they think are real babies to what you’re saying your mom is doing.

His long has it been since she was to the dentist? Could she have a sore spot or a bad tooth? She could cause herself to have a sore spot if she keeps rubbing the same spot; not to mention it’s not very sanitary to keep putting her hands in her mouth. Could she be anxious about something? Anxiety can cause these behaviors as well. If she checks out and nothing is wrong, why not try a fidget spinner for her?