Jacked? Damn straight, have you seen him with those weight lifting scenes?! He lifts it up like it’s a feather!
Dude spends every week running for his life with a dog that’s got to be at least 100 lbs in his arms. Constant aerobic workout with weights, massive hunger, laid back attitude: he’s an fighting-anime hero whose style is entirely built around escape and speed.
According to season 1 episodes of Scooby-Doo, Where Are You, Shaggy is both “the swingingest gymnast in school”, and a track & field athlete who is the fastest (human) in the gang. So, yes. Definitively canon.
His appetite isn’t about being stoned, it’s because he’s a teenage jock.
To be fair, he can be a teenage jock with very high caloric needs and also be constantly stoned. They’re really not mutually exclusive.
Whilst some are wondering what Shaggy is wearing, I’m wondering if Fred is wearing anything at all, other than that conveniently placed surfboard.