Unless otherwise specified, my mosaics are intended for indoor use only. I use Weldbond more frequently than any other glue. If you buy this product, call the phone number listed on the container and request the full spec sheet. It contains a lot of useful information about other uses. Front-glued mirrors cannot be replaced without ruining your mosaic.
Mirrors can break, or they can deteriorate over time.
The primary value of a mosaic-decorated mirror is the mosaic, in my opinion, not the mirror. I find that regular glazed cermaic wall tiles, the kind with the soft white chalky backs, are easy to cut, but a disaster to shape. The glaze is harder than the body, and cuts tend to result in unwanted beveling. Plus, those tile bodies are not waterproof. There are a lot of good mosaic books on the market.