What do you do when a single player has a strong personality and tends to handle the heavy lifting of roleplaying?
Do you cry foul and say "Give the other players a chance!". Or do you just let group dynamics work as they are going to work, focusing on the shy players as needed, but not penalizing the exuberant players when they take charge?
The moon circle druid is not equivalent to someone doing a lot of roleplaying while everyone else sits back. It's more like the mechanical equivalent of one of the players yelling over everyone else constantly when they're trying to speak (at second level, at least).
That's not to say the player is at fault. In my game, the guy picked moon circle because it sounded cool. He was horrified by what happened after. All we could do was laugh at a certain point as the bear just nullified the entire combat pillar of the game.
Interesting. As a player, I play a Wizard. He just made level 4. Total in the campaign, he has done about 100 points of damage in about 16 encounters. I literally roll that bad (even when he hits with a cantrip, I often roll 1 or 2 points of damage) and the fact that most spellcaster damage is wimpier than most weapon damage doesn't help. Several of the other PCs have probably done about 500 or more points of damage in those same 16 encounters (probably much more for some PCs). Even rolling bad damage, they do a minimum of 4 or more points of damage when they hit.
If I took your POV here, I would conclude that Wizards totally suck. They waste a spell every day on Mage Armor, or they risk getting hit even more often then they do. Their hit points are weaker than everyone else's. Concentration totally limits spells now. Their cantrips and weapon attacks are subpar. Most non-instantaneous spells with a save last 0 to 2 rounds.
I would disagree that low-level wizards suck. They bring a lot to the table that other classes don't. In combat, they have the ability to frontload large amounts of damage, and they excel at target-selection. Frontloading is important because it can remove enemies from the fight before they get a chance to do much, and target selection is important because it lets you take out the most dangerous targets first. Wizards can also use control spells for action denial, which can be extremely useful. The total damage you deal is less than a fighter's, but you deal damage to the right enemies at the right time, which is an important role in the party.
Wizards also bring a lot of non-combat utility to the table, with a great deal of versatility to decide when and how to use it. Damage is only a part of balance, albeit an important part.
The problem with the moon circle Druid is that they have all the advantages of being a spellcaster, plus they can turn into an animal and wreck everything. They really don't give up much for it.
But one has to view both the pros and the cons. Does the Druid have any magical items? Most of them do not work while wild shaped. Even a ring. He cannot cast spells, so if he shapechanges back to cast a spell, that's it for his shapechanging until his next rest. If a group of non-mook NPCs storm the bear Druid, they will hit 75% of the time when he is in Bear form and they will sometimes take him down before he can shapechange back. The same non-mooks doing that to the 2nd level fighter who has 60% of the Bear's hit points, but with 5 (or more) better AC will only get hit 50% of the time. They will still take him down too, but the number of attacks should be similar. As for the wizard, he only shines one encounter in three, but he still does more to save the day than other PCs. An infrequent pro to offset his cons.
It really doesn't matter what the bear's AC is. He could have an AC of zero and still be overpowered, because he gets 35 free hp twice per short rest. In certain cases, yes, NPCs will be able to take him down before he can shape change back, but those are scenarios where any other PC of his level would be completely demolished (or are the result of miserable rolls, which can kill anyone in this game). You can always give the players an unwinnable encounter, but that doesn't make it balanced. It just means the moon druid and nobody else will get to be the one with a pile of bodies around them when they go down.