If Mewtwo is a clone of Mew, how is it that Mewtwo comes
before Mew in the Pokédex?
MIND BLOWN.
I always found that weird too xD But the explanation I heard is that Mew was originally not meant to be a pokémon possible to find in the games. Just a mirage, a legend leading up to Mewtwo. In a late stage before the release, they added in Mew anyways and thought up an event for it to be distributed. In the pokédex, it became #151 because it was added last. Maybe it was simpler that way.
Outside of the games, you could maybe explain it with the professors deciding the order in the pokédex finding out about Mewtwo before they got proof that Mew still existed in this world? The fossil pokémon are in the pokédex too, but they could be resurrected (without weird superstrength clones) so that's not so weird.