> This is a boring legacy tech company who’s best selling point is being a discussion forum for the chronically mid.

This insult isn't about the comments on Reddit, it's about the people on Reddit. "Chronically mid" means that the people are average and will always be average. The comment is arguing that Reddit is low-value and part of their argument is that the people using Reddit are low-value "chronically mid" people. Everyone can judge for themselves whether downplaying the value of millions of people is offensive or not.