Well I would say its more of Tony is very quick and agile, just not fast in a straight line. We have seen this on full display over the years, even the nightmare in Seattle years ago. He does have good quickness and agility in the pocket to avoid rushers, but don't ask him to actually run anywhere cause he is pretty slow.
Guess it depends on your definition of mobile, its not like we will be running read option with Romo. Does he have good movement in the pocket, yes, is he going to sprint for a 93 yard TD like Pryor, no. Luckily, you don't have to have sprinters speed to be a QB, and Romo is definitely not a Bledsoe.