While they do have a LOT of the original talent on What If.......there are about 20% of the cast that are NOT the original actors. Most notable for me was Drax not being voiced by Dave Bautista, when asked on Twitter he replied that he was never asked, which I find hard to believe considering how many others ARE a part of the show. RDJ, Scarlett Johansson, and Chris Evans also did not take part.
The guy who played Drax sounded just like the character. Lake Bell, who is a vocal god, does an amazing ScarJo impression.
But, weirdly, a few of the "actual" actors didn't sound "like themselves" to me, notably Sebastian Stan, Dominic Cooper and Neal McDonough.