If Sub-Zero became Plain Zero, doesn't that mean he improved?

It's better than being called a "Christmas Tree", or being bisected with his own chainsaw, or "Buzz Saw", if you will. And at least he wasn't mistaken for Jesus Christ ("guess again").

Well his power comes from his ability to blast people with ice of sub zero temperature, which can be very cold. Plain zero temperature, I mean we've all experienced that, it can be cold but it's certainly survivable. So his attacks would be less effective.