So yeah it makes no sense, but, if Mewtwo had been #151, your Pokedex in Red and Blue would have a gaping hole in it at #150. Mew was considered extinct when the Pokedex was made. But as Mewtwo was alive and known, he was placed first.

Is Mew the 151 Pokémon?

Mew's number in the National Pokédex is 151, the last of the first-generation Pokémon, with 150 being Mewtwo and 152 being Chikorita.

Why is Mew not in the Pokerap?

While it contained almost every Pokémon of the original 151, they left out Mew, who is the only Pokémon to be left out of the Pokérap and no reason has been given as to why. It was only in the Hoenn Pokérap that Mew was finally included, two generations later.