Have you tried temporarily hooking up the Pi to a computer monitor? Verify that the Pi is outputting a signal, then try a different HDMI input on your TV.

I have also found that connecting everything and setting the input to the appropriate plug and then turning on the Pi is helpful. You’ll get to see the Pi boot sequence which eliminates any issue with the Magic Mirror interface/electron not displaying content.

I just tried with the mirror I took off my MM, it doesn’t make much difference if it’s stuck against the display or if there’s a gap, you can go for it ;)

And honestly, I prefer without mirror :D