DPOA was signed by elderly when she has dementia and no knowledge of what document she signed. Is this legal?

I am assuming it was properly witnessed and notarized?
Your mom only needed to show that she understood what what a POA was and that she knew what she was doing at the time she signed it, but if there is a concern that she was mislead or coerced in some way then you may have grounds to question it's authenticity.

lpastrana, did your Mother have someone else as her Power of Attorney in the past?   Or was this a last minute thing when a relative realized that no Power of Attorney had been signed?

Was the POA prepared by an Attorney and the signing took place at his/her office?   Or was this a POA printed off the Internet and signed by your Mom and the person assigned to be POA in front of a Notary?

If your Mother still has clear enough mind to appoint someone to be her Power of Attorney and she understood what was involved for a POA, and signed in front of an Attorney who felt she was able to do this, then yes, it was legal.

It depends on whether they knew what they were signing and intended what it says. Was someone trying to get authority to get them medical care? If you don't have it, it's very challenging. Then, you might have to go to court and Petition for Guardianship. Is there some reason this is going to work against the person who signed it? Are you contending that someone else should be the HCPOA?