I never played the old Monkey Island games in their day because for some reason my dad had pretty much the rest of the SCUMM catalog except for Monkey Island and LOOM. but if I understand correctly, the transition to 3D wasn't as criticized because everyone was trying to figure out how to make games look good in 3D at that point, plus Ron Gilbert had left LucasArts at that point in time, so those games weren't as appreciated as the first two. the Remastered version of the first game was certainly met with mixed opinions though.
> Well, which older game should it match?
I already explained that I agree with you that this is the exact problem they faced when coming up with a look for the game. if they had picked any of the different styles, there would have been fans and detractors both. arguably, if they wanted to minimize backlash at the expense of everything else (such as broad marketability), they would have painstakingly recreated the art style of the first two games, with its weird disconnect between the cartoonish character sprites and full-screen realistic-looking closeup shots intact. so they did the almost certainly right thing and figured out a new style. but just because they chose the correct course of action doesn't mean that it wasn't going to frustrate just about everyone who looks at it and says to themselves, "well Monkey Island looks like many different things to be but it doesn't look like that." as a comparative example, see the reaction to the art style of the remaster of The Legend of Zelda: Link's Awakening. I haven't played it but I think it looks great—much like the Return to Monkey Island art style, it found a way to split the difference in making a new graphical style that still retains some of the "vagueness" of the old, hardware-limited style, while looking unique and good. as you can imagine, even though the Zelda franchise has had its fair share of totally different art styles over the years, there were many people who felt like this particular style doesn't jive with their franchise preconceptions. (I'm kind of at a loss for why I'm fully explaining all of this... this all seems pretty self-evident, for self-evident reasons.)