The tier is well prepared for both fast, strong fighting types (Iron Valiant) and bulky, powerful fighting types (Great Tusk), so Zamazenta isn't going to force team building to adapt to it - the meta is already adapted to its general threat. Being both fast and bulky does mean it's an extra challenge, but lacking a second STAB also means it's not strictly better.
Fighting as a defensive type is possibly the best it's ever been, given the dominance of dark types and the lack of birds, but there's a few new flying types coming in with Home to help check the doggo. The dark resist is likely to remain extremely valuable, though, and a rocks resistance will never go out of style, so Zamazenta's typing isn't a weakness defensively.
If it had any of the good offensive boosting moves, I'd say send it to Ubers without a doubt, but you're left with Howl or ID/BP. It's just about physically unbreakable the first time it comes in, and that might be too much for the tier, but it's not so overbearing that I wouldn't give it a chance. For what it's worth, some of the best checks in NatDex are Gen 9 mons, so they're both available and not relying on moves lost to dexit.
