Can mirrors block my wifi signal?

A (fairly modern) mirror shouldn't block your signal.

If there are two mirrors between your bedroom and the router: is there a bathroom, too? If that's the case then i would suspect that the tiles on the bathroom walls are one of the factors leading to your signal loss.

Is the building you live in build from armored concrete (like it's usual in Germany, for example)? Then you'll have another big factor.

It's conceivable an older mirror could cause issues. The glass shouldn't matter at all, but if they are heavy backed (silver or lead) it could be a problem. You're more likely to experience signal interference from other wireless sources like additional routers, cordless phones, etc.

It could also be the walls, but it would depend on what is in the walls. If it is a wall with a ton of plumbing and wiring, it could effectively block a wifi signal.

In a new bath room, much of the plumbing is plastic (PVC or ABS). in an older building it would be copper and cast iron, which will block more of the signal. Bathrooms also tend to have more wiring then other rooms which could block more of the signal. If the walls are tiled, they not only have the tile, but usually have a heavier cement based backer board which will block more of the signal then regular dry wall. Also don't forget the tub. A large cast iron tub would definitely block part of the signal. A fiberglass one would not block as much.

Finally as for the mirror, the silver backing can be expected to block the signal. On older mirrors the coating was actually silver which is an excellent conductor, meaning it would block the signal. Newer mirrors use a thinner silver colored paint usually with at least some aluminum in mix. It will still block at least some of the signal, but not as much as the thicker silver.

In summary, a new bathroom will not block as much of the signal as an older one will.