The "receivers break out in Year 3" used to be the rule of thumb among wide receivers. I used to target 3rd-year receivers in fantasy for that reason. Braylon Edwards might have been my biggest hit out of that group.
I don't think that's as big of a deal as it used to be 15 years ago. Mims needs to make an impact in year 1 and year 2. I'm not expecting 1,000 yards, but if he can have 500-600 yards and 5-6 touchdowns, that would be fine, as long as Perriman and Crowder do their jobs.
There will be a learning curve for sure, but even if Mims is used in a bit role like a Tre'Quan Smith, he needs to make some plays this season. Smith had 28 Rec, 427 yds, 5 TD as a rookie in 15 games.