Zamazenta is fine. The real issue I see with the meta right now is Booster Energy. Even since the prehome meta, mons associated with this item have been an issue. For example, the most egregious offenders (legal for longer than a week) were Iron Valiant and Roaring Moon. Both of these mons had very viable calls for a suspect at the time because Booster Energy raises their speed and attack to ludicrous levels, respectively, compared to the rest of the metagame. Next, we had Walking Wake drop, and the meta scrambled to find checks before we thankfully got one or two in DLC (prim, raging bolt). Fast-forward to now and we have Walking Wakes, Gouging Fires, Raging Bolts, Iron Moths, Roaring Moons, Iron Valiants and EVEN GREAT TUSKS boosting their stats to levels previously-reserved for a mon that had to be choice-locked. Meanwhile the meta still is struggling to cope with these threats. According to usage stats, the most common Walking Wake checks are Blissey/Clodsire (mostly reserved for stall), Roaring moon, and Raging Bolt. The most common Raging Bolt checks are similar with the addition of steel/grounds Iron Treads and Excadrill, both of which are suboptimal mons to run and get easily countered by weather ball/terablast sets. Iron Moth, a mon that was UU until recently surging back in popularity with Sub and other techs, gets checked by dnite and the aforementioned special stall walls. Roaring Moon trades for at least a mon against BO/Balance and can outright sweep most offenses without a dedicated answer.
If you aren't packing one of the specific checks I just mentioned, each of these booster mons can set up in your face and just sweep. Without the added power of booster energy, they become much easier to prep for and play against.
The most common argument for keeping Booster Energy in the meta I see is that, since it is a one-time boost, just force the mon out! My response is this: there are very few options present in the current metagame that can force a boosting paradox mon off the field. Booster Spatk Raging Bolt clicking CM on your offense team? Better hope you have a booster speed ival with encore. Booster Attack GFire/RMoon clicking DD on your fat team? Better hope you're packing unaware Dozo or roar Zama. Not to mention these booster threats can tera to beat most of their checks.
Another point I see defending Booster Energy is that its a balancing force in the metagame. "How am I supposed to check these fast threats on offense without booster ival?" Use a scarf mon like God intended. "How am I supposed to break stall without booster gouging/bolt/rmoon?" Use one of the multitude of other choices the metagame offers.
Now that the meta has settled into something of a (albeit tenuous) balanced state, I think it's time we re-evaluate some of the things that make this gen so swingy, such as Booster Energy. Combining booster and tera exacerbates the issue, but since the playerbase likes tera so much, I won't bother getting into it.
Edit: I agree that Zamazenta is a potential issue in the meta, but I think a booster suspect would be more impactful in terms of making a positive change to Gen 9 OU.