Depends on the wavelength. Standard mirrors don’t always do mirror things at wavelengths far outside the visible spectrum.
Part of the advantages of UAVs is that you can deploy a lot of them cheaply with stuff you buy on eBay. While eBay does sell some of the more exotic mirrors for CO2 laser cutters (which are far-IR wavelengths), you couldn’t buy a lot of them to cover a single drone. It’d cut into the cost advantage, and would also weigh it down a lot.