Every team should fear mediocrity, but a mediocre team in my opinion is a team that barely makes the playoffs every year, or just can't get in. So a team in the 12-17 range. These are the draft picks that typically are not good enough to help elevate the team into the next "tier".
However one common thread for most mediocre teams is a team with a bunch of slightly overpaid vets. If a team has a lot of youth, and flexibility to add assets (via lop sided trades, or free agency) then those teams can still become better even if they are in that dreaded 12-17 range.
I am very skeptical of BC, but he has a team of youth with flexibility. The core is gradually being built from the ground up (Casey/JV/Lowry make a nice start) and the core will be added to every year either organically (DeMar/Ross/Fields) or through future transactions. And one thing about BC is that he likes wheeling and dealing. I just regret that we couldn't move into the 1-7 range last year since those were potentially the best picks available, but c'est la vie.
Last year the team was in 8th (actually tied for 8th) worst, and the year before that 5th worst. That's a good spot to be in. If their goal is playoffs this year, then that's fine because I like part of that core. If they continue to just make the playoffs or end up 9th/10th/11th, then its something to worry about, but we have not even scratched the surface of mediocrity, so in my opinion it's too early to talk about it.