There are plenty of Internet sources claiming that Mark Twain said this, but I couldn't find it in any of my usual, definitive sources (including Wikiquote).
The closest I came to a confirmation was in an article on the website of Time magazine, dated 15 June 1970: "After he visited remote Mauritius in 1896, Mark Twain quoted an islander as saying: 'Mauritius was made first and then heaven; and heaven was copied after Mauritius.'"
So if Mark Twain did say this (and on Time's authority, I'm prepared to accept that he did), he was himself quoting an unnamed Mauritian islander. So although we may infer that he agreed with the islander, we don't actually know what his own opinion was. To describe it as a quote from Mark Twain may therefore be somewhat misleading at best; and this may be why I didn't find it in any definitive sources.
Even so: as (a) it's described as a Mark Twain quote on several websites, (b) he does appear to have spoken the words, and (c) it's come up at least once in a quiz to my knowledge, I'm prepared to give it space on this website.
© Haydn Thompson 2020