It’s better to practice in an open room, so that you can see what it’s like to throw your energy around, look in many different directions and imagine your audience. And better yet, get an audience whenever possible, even if it’s your roommate, friend or spouse! And look for opportunities to practice speaking in front of real people - there are a million low-risk venues like senior centers and kids’ clubs that would love to have a speaker. You’ll get used to feeding off of their reactions and energy and get precious experience at the same time.
You want to know what it’s like to actually stand up in front of a crowd and how much to project your voice— experiences you can’t get from your mirror.
It’s a much more authentic way to rehearse, and it will supply you with real feedback.
So put the mirror away, get on your feet and practice often and out loud.