Visibility of system status is one of the 10 heuristics defined by Jakob Nielsen, even though Mailchimp does a good job when it comes to the wording, the screen you uploaded is not primarly meant to make the user "happy", the primary use is to tell the user that his action was successful.
You can read more about that here:
10 Heuristics defined by Jakob Nielsen
So if i would have to name this kind of screen i would name it "feedback screen".
Many companies with a different language defined by their brand guidelines would not use the "high fives" as the headline and instead would write something like "Success" etc.