My brother topped out low 80s probably sat high 70s-80, but had a filthy hook, nasty split, a great change, good movement on all his pitches, and could locate all of them. He was a very successful D2 pitcher and had the opportunity to play D1 when entering college. I also have another friend who threw 96, but was a terrible pitcher. Had no other pitches and no control what so ever, literally could not hit the strike zone for his life. He didn't get to play in high school because he'd throw 16 pitches and the first run would be walking home and no one would be out. So he walked on at a D3 and did terrible there, had an ERA over 4. He did get drafted after that for the velocity but the moral of the story is that velocity alone is a tough determinant, there are too many other factors that come into play.
I also don't think 88 - 90 something is the D1 average. If I had to make a guess I would say on average, D3 is probably somewhere in the 70s, D2 high 70s - low 80s, D1 mid 80s. There's people who throw faster & slower in all of course but thats my guess towards what the average velocity would be in each division if you actually went to each school in the country, good and bad, and actually tried find the average velocities. Thats just an absolute guess though lol.