I work as a Deputy Returning Officer for elections in Canada. This role (simply stated) involves distributing and counting ballots at a local polling station, and ensuring the Elections Act is followed. While I do not support disenfranchising the mentally disabled from voting, it should be recognized that often its a parent, spouse, or caregiver who is the one voting and making the choice for the mentally disabled voter. This can mean they are essentially voting twice, or more if they are assisting multiple people. In one case in the last federal election, I refused to allowed a man to assist both of his children to vote, instead his wife had to assist the second child. This was within the authority of the role and there was no problem.

As long as there are some safeguards in place the mentally disabled should be allowed to vote, or make the attempt. If you sat there for a few days and saw who came out to vote, you may rather wish they too weren't having an influence on helping to elect the next government or opposition parties.