Not an expansion; a different game on the same engine and setting. There are minor mechanical differences, and M!M! is separate standalone corebooks in all editions.Monsters! Monsters! It was an expansion for Tunnels and Trolls.
Also note: When Flying Buffalo was sold after Rick's death, M!M! and T&T became separate IP's. Ken owns M!M!, and it's no longer on Trollworld. Ken no longer is involved with T&T proper.
Source: Ken St. Andre on Facebook.