I dont know if mentally incompetent is a specific termn in english, but if its simple about people that are not allowed to sign contracts on their own, so being declared as mentally incompetent, doesnt have to meen that this person is complete insane. So when my grandaunt got older and got weird, my parents also declared her to be mentally incompetent. It didnt mean that she was totally crazy or whatever, so 95% of the day she could do just fine and sometimes, whyever, memories or her brain refused to work for some minutes or a hour and she did something completely weird. So when she signed again some contract for internet ISDN (hightech around that time) when she didnt even have a computer, my parents declared her to be mentally incompetent, simply because if she signed a contract from now on, it was not guilty if my parents or her "mentally ill person carer" didnt subscribe it either. So she could go on as she was used, but if my parents found out that some company tried to trick her, they simply could undo the contract. But the gross of the time she was mentally functioning, still able to live on her own with some support from us and so on. So I wouldnt know, why she should have been forbidden to vote, only because of being weird 5% of her time.
So it later became worse, but around that time you could said that she didnt know about what she was voting about, she also didnt realize that there was a vote, so then she didnt vote anymore anyway.
I thought that specially around this forum, more people would understand on their own, that being mentally ill or incompetent isnt linked automatically with being complete GAGA and painting pictures with your poo. ^^