It's clear OP means overrated as an opinion. That can be interpreted in many ways and I'm having a hard time seeing how it could be offensive.
Saying something is overrated is a very shallow description. Personally couldn't be bothered by statements like that about something I'm passionate about.
Either way, saying something is "overrated" isn't necessarily pretentious. It's obvious that in most contexts, the speaker is voicing his own opinion against the crowd.
The OP even says very clearly, "In your opinion, what are some of the most overrated anime, and why?"
It baffles me how this can be interpreted as "people thinking there is some objective way to quantify how good a show is."
To be clear, I'm not disagreeing with your points. I just think it isn't very relevant in this thread.