Can muons decay into quarks?

No, muons can't decay into quarks because quarks are confined; the final product cannot be quarks, but rather composite particles made of quarks, such as mesons and baryons. The lightest mesons are the pions, which are already heavier than the muon, so any such decay is forbidden by energy conservation.

On the other hand, the extremely heavy tau can and does decay to light mesons quite often, as you can see in the PDG entry.