No, it isn't. It might be unconstitutional if mandatory voting required you to choose between a fixed number of candidates, but there's nothing stopping voters from putting whatever they please on their ballots.
You are legally compelled by the government to fill out tax paperwork every year. Being (much more lightly) compelled to fill out a ballot is no different.
You are legally compelled by the government to fill out tax paperwork every year. Being (much more lightly) compelled to fill out a ballot is no different.