Not the right location for a mirror.

Doesn't complement your current furnishings.

Sliding panel color is too creamy.

I think something more vertical - taller than width (rather than square) would possibly fit the space better?

I agree it's not the right place for a mirror, and whatever goes there should be much smaller so it doesn't look squished between window and door. I'd look for a small framed print that echoes the piece over the sideboard.

maybe a wooden piece of some sort that matches the top of the sideboard?

Agree with others - not the place for a mirror.