Mariota could add some weight (much thinner than you want a QB - especially one that wants to run as much as Mariota), but he has really nice physical tools.
Two things would cause me (not speaking for the Dolphins - but in general) be uneasy about drafting him high (top 15):
1. He's a mild-mannered guy. Shy. I know Oregon is a high-profile team these days, but most of the time, they're the far better team. Does he have the personality to lead a team through sustained adversity? Every QB has times in his career (usually early) when his team is stuck in neutral. Leading by example goes a long way, but is he the kind of guy who will hold his teammates accountable?
2. He's going to be playing a completely different game in the NFL. Even if the Eagles draft him, he's going to have to hold the ball more and go through reads in the NFL. Teams are going to make him a pocket passer, and they're not going to give him quick reads all day - like he gets in the Pac 12.
Usually, people think of the interview process being important for guys like Coples and Jenkins (problem kids), but I think it'll be as important for Mariota as anyone. Teams will want to see if he has the football IQ and the leadership qualities to play in the NFL, because you're making all kinds of projections with him.
I would not draft him ahead of Manziel; I would draft him ahead of Boyd and (probably) Hundley.