You definitely have to factor Muay Thai training as a workout. Especially since it usually (depending on your school) involves lots of pad work, bag work, and jump rope. I wouldn't mind considering it your main form of cardio.
That's a big problem I hear about, people think they need to go to their classes, lift weights, and still find time for G.P.P. or whatever. Unless you're noticing a significant shortcoming in your in-ring performance, lift properly, recover properly, and work hard at MT, and you'll be fine.