It doesn't matter. More importantly for him, he was a project coming out of HS. If he's smart, he should want to go somewhere that he'll be coached the best, which should put Miami in the driver's seat. Jermaine Johnson already had a few years of elite coaching under his belt, he was just stuck behind kids who were even better than he was, so he decided to leave. Mims is stuck behind kids who are better than he is in a similar fashion, but he's nowhere near the "plug and play" prospect that Johnson was. He needs to be coached.
That being said, were he to continue to develop, and reach his potential (big "if" under Atkins), he could and would absolutely stand out at FSU regardless of the guard next to him and Jordan Travis, etc. Does it help to play surrounded by a bunch of All-America's? Sure. But it really doesn't matter if you're truly a first-round talent.
And running out of time? He's 1 year out of HS. He can't even go to the NFL until the 2024 draft, at the earliest. He's got 2 years to put whatever he's going to put on tape. Jermaine Johnson was running out of time, I'm pretty sure last season at FSU was his 5th out of HS.
Lastly, the "****** OL" argument is a year or two expired. FSU certainly doesn't have a great OL, but those years of being the worst in the country are in the past. They actually had #'s 1 and 2 in YPC last year in the ACC (believe it or not, I was shocked when I noticed it at the end of the season), and their offense actually averaged more yards per play than ours did. Travis is a ****** thrower and their receivers are overall trash, but they ran the ball pretty successfully last year and the OL was much more competent than it's been in years past. There are several reasons why Miami would be a better choice than FSU for Mims, but the "not standing out" or players around him argument isn't really one of those.