Have you ever played a good Grodd?
I have, and I play Grodd myself.
Not going to say my word is law though, I'd be estatic if Grodd was considered mid tier, but no one, played against or watched playing, has convinced me otherwise.
Edit - Look, if you can give me actual information about Grodd's tools and why that puts him in the same tier as characters like Atrocitus, then I'll be ready, but so far all I've seen is a couple of Grodd matches against people with an obvious lack of MU knowlegde and people just going like "Wow Grodd is actually good/has a vortex/does mad damage" which has nothing to do with why people consider him below average. Not even to mention is horrendous matchups.