The problem with Brendan is that he's simply not likeable, relatable, unique or at all interesting. The guy is not particularly bright or insightful, and on top of all that it is known that he got a leg up on the business through his connection with Rogan and his UFC/TUF TV background. In a way, he is to comedy what CM Punk or Greg Hardy were to MMA. A sort of novelty act. As such, it's expected that people are going to be tougher with him than normal.
We can even look within MMA to see a different case. Compare him with someone like Derrick Lewis, who is likeable, unique, legitimately funny, creative, original and has natural charisma. People would actually clamor for him to do standup because he comes across as authentically funny.
The people that spend tons of time trashing him online in elaborate ways are weird, but I can see how he's an easy target. He has thin skin and is easily gotten to, he brags and cares a ton about his appearance, clothes, shoes, money, mma career, etc. He's also not particularly talented at comedy, but maybe not much worse than your local small club journeyman comic. The difference being that he gets spots at top clubs along top comics because of his fame, which came by through unrelated activities and connections.