Considering that in Stellaris, all pops can have exactly one ethos, I think it would make sense for the Hive Mind "ethos" to cost 2 ethic points rather than 3, and for all pops in the hive mind to always have the remaining ethic you select.
That's a little different though, pops represent a multitude, and you can assume that the pop ethos is just the most important one to a significant portion of the population. The population as a whole surely has positions that are all over the map, and constantly changing at that, the only reason the ethos stays constant is because for every pop that moves away from it there is one that moves toward it.
Similarly, government ethics represent an outlook or a set of policies that have cemented themselves in place. With Hive minds though, you essentially have one individual that can change their mind and the direction of government and society on a whim. Even though the hive mind might hold to a particular ethos, this liquidity demands a different sort of representation in the game mechanics - or at least I think it makes sense if it does.
The other concern is balance, I think. It wouldn't make sense for Hive Minds to put any sort of policy restrictions on themselves, and the bonuses you get from ethos aren't designed around a hive mind society, so some might be super useful and others close to useless. I suppose you could customize the bonuses from ethos to work differently with Hive mind, but I can understand the desire to treat Hive Mind as a holistic system and just balance one permutation against everything else that already exists.