Is it okay to have a mirror in the secret room?

You can't. Because avoiding spring requires you to be alone in a room, draw the curtains and you can't see any light. Mirrors can reflect or refract light. Hiding in the spring is just to avoid the first period of the arrival of the solar term in beginning of spring, because beginning of spring is the time when people often say that putting one's foot in the door is handed over, and at this time, people who commit putting one's foot in the door should hide to avoid putting one's foot in the door to bring themselves bad luck and good luck for one year.