I tend to flip flop back and forth on this. I wouldn't say that it's related to Phil's stupidity, as much as Phil's ability to live completely in denial.
At the risk of sounding like an armchair psychologist, I think Phil's hermit lifestyle has definitely skewed his sense of reality. He's by no means full on crazy or hearing voices or whatever, but I think he's lived in his own bubble for so long and dismissed all criticism, that he actually believes some of his lies. Mostly about opinion based things, something factual, like Line or Champions, that he knows he's straight up lying about. But, him being mature and successful and everything, well, when everyone in your life tells you something is true for several years, you're bound to believe it. And Phil has made it where that's his reality. Yeah, he has detractors shouting at him about being a failure, and I do think in some gin fueled moments he actually reflects on what's being shouted at him, but otherwise, he's deluded himself into automatically thinking they're wrong and just jealous trolls.
He shows a lot of signs of this whenever he talks about anything real world. When he talks about expecting to get attacked while shopping or Walmart being an IRL Call of Duty match, he's being serious. Same with his excitement when the most mundane of things happens to him, like someone coming to check his alarm. He legit thinks it's a huge deal that requires 30+ minutes of explanation because how could anyone grasp the complex intricacies of a monumental event. Same deal with him insisting Jasper is actually his family, or his crazy theories about lag match making.
Someone made a post last year, when we were a good five or six months into quarantine talking about how much the isolation has messed with their general perception, and how it's crazy to imagine that, with the exception of his one day off a week, Phil has lived like this for years. And I imagine that person was still socializing way more than Phil does on average. Deep down, I think Phil knows he's a failure, but without anyone to hold him accountable, he has plenty of ways to hide from the truth. To sink even deeper into the armchair, I imagine that's part of why he's addicted to mobile games, because they provide a constant dopamine hit and tell him that he's winning by leveling up his wrestlers and unlocking new characters. He's constantly chasing the win, whether in mobile games, his "savage viral tweets", lean in banning someone or "going beast mode" against scrubs after tanking his profile low enough to be matched up with brand new players.