As someone who's actually replaying the BG series right now, I think both games are pretty campy. And there's humorous tones inserted into all sorts of other NPCs. BG2 is slightly darker than BG1 (which I would say BG3 is closer in feel to). But still, any thing that comes out of Jan's mouth is wackier the BG3 companions.
Really, I think the reason why people feel such a distinction between BG1/2 and BG3 is the speech pattern used. BG2, despite having wacky moments, uses "Fantasy English" in their speech. So even though a storekeep maybe making a joke, there's a lot of "Alas" and "Ay"s that keep it in the genre of conventional fantasy. Whereas, BG3 in the most cases much closer to modern day people (especially with the VA) - so when it's a joke, I can see how it can really take you out of it.
And I think not having voice acting in many of these jokey moments help too. For example, one of my favorite "fun" interactions from BG2 is this one:
Quote
Main Character: What is it, exactly, that you guard?
Spectator: Just the one chest behind me. You are welcome to open the others with the imps, or do whatever else... I'm not here to do anything about them in the slightest. I think the Sahuagin looted the other chests long ago. They might have put some stuff of theirs around here, though, thinking I would scare off thieves.... oh, that reminds me. I promised that mad little Sahuagin that I would make an effort to scare off intruders...
So... boo!
And that's about as much effort as I'm willing to put into that. (sigh) It's been a pretty lonely experience so far, you know.
Reading that, you can still keep the tone of the world in your heads, despite the Spectator speaking like a guy doing a night shift at a 7-eleven. With voice acting though, I feel like that illusion will shatter.
