This seems like an apples to oranges sort of comparison. The world of Diablo is significantly darker and more horrific than Forgotten Realms DnD and that sort of art direction really wouldn't work here. While the original BG games had moments of darkness they were nowhere near as dark as Diablo. The art direction of the classic BG games was pretty standard fantasy fare with a hint of darkness at times, which isn't bad by any stretch (it is accurate to FR after all which is very much standard high fantasy) but it isn't Gothic Horror like the diablo games.
Personally, I find the art direction of BG3 to be pretty good so far since for the most part it seems like a pretty good adaptation of the Forgotten Realms setting. In fact, my only complaint would be that the grittiness feels a little overdone at times but that just might be due to the graphical detail of all the gore.
The immersive environmental effects in Diablo are cool but it's still not really a fair comparison since there's probably loads of technical reasons why similar effects aren't used in BG3. It's also not really a fair comparison when it comes to budget since the one for Diablo 4 is probably much higher than BG3 due to all of Blizzard/Activision's funding.
