Money; that's usually the reason for lackluster endings such as the one for CMI. It was extremely rushed, had no emotional climax at all (in direct contrast to LCR). I read an article on World of Monkey Island, a very insightful one where the author speculated indeed whether the whole of the two games was meant to take place inside Guybrush's imagination, and that his brother Chuckie trying to find him inspired the character of LeChuck in his mind.
This culminated in Chuckie chasing Guybrush around the staff tunnels at the supposed park; Disneyland has these exact kinds of tunnels. And it makes sense when one notices that Guybrush can take an elevator up to Melée Island from the first game. Chuckie did a LeChuck expression at the end and Elaine was seen still standing above the treasure pit to, according to Ron Gilbert, keep the door open for sequels.
As for Big Whoop, I thought the article author made very much sense. If you pick it up in the Underground Tunnels, it's just a ticket. Apparently it's one of those 'free pass' tickets that you can get at Disneyland, and a thing that surely must have been exciting to a little boy.