If a character is speaking and it sounds like a well composed post on a forum or something like that, it can strike some people as fake. Then there's the additional layer of difficulty in keeping consistent with what we know of that character and their circumstance, but without making it feel like their dialogue is aimed at defining their character and avoiding really one note/stereotype style characters(I'm feisty! I'm gloomy! etc. etc.). We should learn what an NPC is like more indirectly.

A great example of a character speaking way out of what you'd expect as natural for them is pretty much all commoner NPCs in Morrowind, who will go into encyclopedic paragraphs about the setting. They had a certain functionality but definitely did not feel like real people most of the time. Bethesda has always had trouble with writing convincing dialogue.

Pillars of Eternity and Rick and Morty also come to mind.

Pillars is often criticized for being overwritten, and I agree. I actually like Obsidian's writing for the most part, but presumably in part because they focused so much on introducing players to a new setting they got carried away in PoE. In general, NPCs just say too little with too many words and redundancy and overly informative feeling sentences become noticeable. Plus the backer NPCs didn't help, I knew what they were and they weren't a major problem for me, but it was probably a bad idea to make those one of the backer rewards. Ah well.

Rick and Morty I just happened to find a topic on reddit where someone criticized the characters for having what others would call more natural speech. In Rick and Morty, you get more of a casual conversation feel. Their dialogue would not look pretty as a forum post or a formal speech, and that's part of why it's good dialogue.

Another example of it being done well, is the comedy show "Peep Show". It is easily in my top 5, possibly my absolute favorite television show period. Watch pretty much any episode of it and you'll see what I mean.