Nah, he just wasn't elite in the NFL. I don't think muscle really hurt him. He weighed the same as all the other 6'3 quarterbacks, and probably carried less weight as he had more muscle than Matthew Stafford, yet he weighed the same. Muscle is very easy to lose, so I'm sure if it was hindering his performance, he could've just eased up on the curls and butt flexers.
Buchner could do a lot for himself as a freshman by adding 20 lbs of muscle to his lean frame, especially in his legs and back. For instance, he is listed at 6'2. Aaron Rodgers is also 6'2. Rodgers weighs 225 and Buchner weighs 200. Rodgers is a frail quarterback and doesn't run the ball. Buchner is more athletic and will be running the ball a lot. He should try and get to at least 225, not fat, but muscle. All of the other 6'2 NFL quarterbacks weight at least 220.
He certainly should spend the vast majority of his energy on running the offense and working on mechanics, but adding thick, solid, mass to his legs, butt, and back can have a huge payoff, especially for a guy that is mobile (less injury, stronger, faster, bigger). If he can add 20lbs to his frame, he could become very difficult to tackle, making him a huge weapon. I've never bought into the theory that muscle slows you down. It just doesn't make sense from a science perspective. Adding fat certainly can slow you down, but theres a reason running backs spend so much time doing squats (they don't slow you down).