Can a notary sign for a document sign of a person not capable to understand what their doing?

Notaries don't notarize documents, they notarize signatures. A notary is simply certifying that the signature on the document is actually the signature of the person it appears to be. The notary makes no warranties or certifications about the contents of the document or what it does. A notary rarely even reads the document. Instead, he/she will look at the signor's identification, watch them sign, and then sign and stamp as notary.