It's an instance that became known for brigading other instances to "dunk" on anyone who has remotely different ideas than they do, as well as spamming oversized emojis and pictures of pigs shitting on their own testicles. They have been defederated by most instances for good reason, as they always showed up enmasse to completely derail discussion and their own stated purpose of federating with other instances was to "dunk on liberals and dismantle Western propaganda". Their users would harass people in DMs for weeks on end if that person said something they didn't like. I was there to see this happen in several instances before they all defederated from Hexbear and it almost made me quit Lemmy entirely.
Below are some examples of various instances considering federating with Hexbear, only for Hexbesr's users to cause enough problems to get defederated. The threads themselves are best viewed from an instance that federates with Hexbear in order to see the awful behaviour for yourself, and the lemmy.ca thread has several more examples of this.
https://lemmy.ca/post/3326347 https://lemm.ee/post/4543536 https://lemmy.blahaj.zone/post/2017079 https://lemmy.world/post/249833
Do note that a lot of Hexbear users have alternate accounts on lemmy.ml and midwest.social (because nobody wants to federate with Hexbear), and a lot of these people will be in this very thread trying to defend Hexbear. Don't be fooled by them. Hexbear is a toxic cesspit.