Would scoring well on a non-required GRE Mathematics Subject Test make me more competitive? I would like to get into a top university for graduate school, specifically in computer science.  Would taking the non-required mathematics subject based GRE test make my application more competitive?  Since it's not required, would the people doing admissions even consider it as a data point?
No one here can speak for some admissions committee. They might look at it, might not. If they have strict rules they might even need to ignore it. It won't do you any harm, though and there is an outside chance it will make some small difference. But focus more on the things that they do ask for. 

And, for what it's worth, the competition at top schools is fierce. Get good letters of recommendation.