Basic Action Super Heroes!

Would it be acceptable within the rules to choose the category of "humans" in the power X Mastery?
It seems like it would be possible. The power says that the controlled people wouldn't directly endanger themselves, so it would be good for "broad" control rather than "deep."
Controlling large numbers of humans to do mundane tasks, rather than one person to attack their teammates.

No. Indeed the original edition actually included the text for X Mastery "X cannot equal people". I probably should have kept that line. It it too potentially powerful

Mind Control can be used to affect crowds of people, as long as they are minions (which would include ordinary bystanders). If you want it to be really wide-scale, so that everyone watching the TV might be subjected to a villain's Mind Control, that would be handled either as a Plot Device or as a Cosmic Power. It generally should not be a power that a villain could use whenever they want to, but only in a rare situation. Otherwise, the Heroes would pretty much HAVE to kill that person / put them in a medically induced coma for life / lobotomize them or do some other ordinarily super-unethical thing just to keep them from doing it again.

Another alternative for you is "Suggestion" with a large area of effect. Suggestion is also less powerful than Mind Control and people won't endanger themselves over it.