Is my mom's personality a sign of pending dementia or just exaggerated by the disease?

Having extreme and dramatic responses to events may have been symptoms of a mental health issue of some kind -- not necessarily dementia -- so in that sense it is likely she did not have full control over her reactions. Perhaps with a diagnosis and some treatment plan she could have gained more control and been a happier person. The same is still true. A treatment plan could help her be happier even with dementia.

Many personal characteristics seem to be exaggerated in dementia -- perhaps because social filters are reduced or eliminated. This doesn't always happen, but it seems common. One woman in my support group stated her husband with LBD had become violent, and he had never been violent at home before. So that was a change. But he was a policeman, so violence was not foreign to him. Who knows how the dementia impacted that? Another woman, married to a minister, said her husband had always had an interest in the welfare of children. With his dementia he had delusions and hallucinations that there were children in their home, and he wanted his wife to feed them at mealtime.

A professional wrestler in this area was always the "good guy" in the wrestling drama, and he was widely regarded as a good guy in his personal life. He was generous to charities and civic funds. He spoke often to various organizations and groups. He developed dementia and was in a highly regarded care home. And in an argument with a frail elderly resident, he picked the man up and slammed him on the ground, causing his death. This tragic event certainly was caused by the dementia. He was not in his right mind. He had the strength and the skills to perform this act, but he had never before used them outside of drama that is professional wrestling. He had full control over the use of these skills in his younger days, but not, apparently, once dementia took over. (By the way, the widow understood that it was the dementia that caused the outburst, and did not press any charges against the wrestler or the car center.)

The relationship between the kind of person one was before dementia and what happens once dementia is in the picture is complex and not fully understood.