Thank you for your thoughts siusiufengshui.
What I've found in my experience with working with Marie Diamond's system and Master Chunyi Lin's Spring Forest qigong over the years is that objects do pick up and hold energy from that which they have been around. And mirrors, even being reflective in nature, holds that which it interacts with.
Sharon - take the time to clean/clear it and it's not an issue either way.
Much Love,
Shawn