It's absolute trash. The characters range from poorly written to completely loathsome. The premise of "awkward, loser shut-in gets visited by the embodiment of perfection in the form of a cute, young girl who forces her way into his life and makes him turn his life around" is contrived, unrealistic, and just plain stupid. The humor is awful, and mostly relies on the inconsistently awkward main character making a fool out of himself as the punchline, which doesn't work because it comes across as pathetic and sad, rather than actually funny. The anime has weird moments where the tone of the anime doesn't match the dialogue, and it's confusing and clumsy, and it's hard to make out any kind of consistent themes out of the story. 3/4 into the anime, where I could not bring myself to finish any episode, I had no idea where it was going or what it was trying to get across. The episodes are cliche and boring, but also ridiculous and stupid at times. There's a ton of filler, and the story drags like hell. The voice acting is poor, the animation is poor, and the only redeeming part of the anime is that the music is admittedly pretty great at times. I watch that opening and it makes me want to watch it again, give it another try, but it's just bad. A massive disappointment. The anime sucks. If anything, read the light novel. I hear the anime dumbs down the story and it seems like a lot of the problems I have with it (inconsistency, tonal dissonance, filler, production values) are the result of a poor adaptation, but I haven't read it, or even finished the anime, but I cannot imagine it's worse.
It's realistic in that the characters are fucked up, and the anime acknowledges it, mostly. It's not like No Game No Life, where they're losers, but the anime treats them like they're gods, and perhaps it's the best anime that deals with mental health problems. If you want that, and don't care how well it's done, by any metric, Welcome to the NHK will deliver, but don't ever try to compare it to other drama/slices of life. If you want a realistic, mature slice of life, watch Planetes or Nana, and don't bother with Welcome to the NHK. The fact that it's so popular on reddit baffles me. Are we really that desperate for some dark and edgy wish fulfillment that we put up with this?