Honestly with regards to fat having been able to get a high number of good reviews I think the answer is simply because the people renting his half-hovel have zero context as to the depths of how much of a retarded sack of shit he is, nor are they ever in the position to inadvertently transgress against his many red lines due to the limited contact beyond fatrick occasionally dressing up as a french maid and waddling round with a cum mop in the aftermath of their soirees
Speaking from personal experience, having known more than my fair share of IRL retards and BPD/psycho specimens, they usually act overly friendly and obsequious and accommodating if they deem you someone they can use somehow, but the femtosecond you trigger one of their tism triggers thats when they go absolutely apeshit either IRL or remotely and will hold a grudge fucking forever about it no matter how jawdroppingly petty the triggering incident was.
Transposing this behavior onto fat, I suspect he would act extremely and almost desperately friendly to his airbnb patrons in their limited interactions and would likely be low-key tard wrangled by Nikkki to ensure he didnt get out of hand, and would thus be treated with a degree of condescending pity, like if they were in a restaurant being served by an autistic waiter who is just a little too enthusiastic about talking about star wars to them but dutifully went back to the kitchen before it started to get annoying, which would almost subconsciously make them forgive his inevitable social faux-pas.
Hell, when combined with how infamously quick and obsessive he is in responding to literally every single communication he ever gets, this would actually work in his favor as he would look downright attentive, and the fact this would make him look like he had nothing else going on in his life to the airbnb patrons would further increase whatever pity they felt for him, and thus make them that much more likely to do a good deed for what they see as a well meaning and slightly retarded manchild by leaving a positive review for him.
Maybe I am just overthinking it, but I feel for us to truly understand the pig we must unlearn all we know of the pig and look at him with virgin eyes once again from a different perspective