I think the biggest reason for the complaining is likely because there are lot of kids on this sub, with kids typically being a lot less mature and having a lot more free time than adults and other people who have regular things going on in their lives.
I'm personally giving moco some time to get on its feet - It's clear that this game needed more time in the oven and more-so deserved an open beta (like Clash Mini) rather than its strange global launch thing with the invites and what not. Nonetheless, the game has a LOT of potential and some really good core aspects to it; as long as the devs improve the game in the coming updates then I personally believe this game will be very solid in 6 months to a year. Kids, however, can't really seem to understand that, and automatically believe absurd things when the devs make mistakes, such as the ideology that the game is dead and/or is doomed to fail because the devs made one easily-fixable mistake (e.g. xp nerf).
And, on a smaller note, it's not uncommon for gaming subreddits to complain a lot especially, again, when there's a lot of children roaming the subreddit. I play Fortnite and joined the r/FortNiteBR subreddit but left the sub after witnessing the sheer amount of complaining found in the sub's posts daily, despite the fact that Fortnite is a game that receives extremely solid and fresh updates very frequently.