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.