I have said this 100 times. There are 100 topics about this.
Mametchi and Mimitchi are no rare, they're easy to get with very good care and playing lots of games for happiness hearts. NO tamagotchi is really rare, except Nazotchi and the debug characters.
Some people consider some tamagotchis rare, because they never seem to get them, but there's people who get them always. That's because different people have different usual ways of treating their tamas. Someone may forget to take care of their tamagotchi and finds the healthiest characters rare, someone may check his/her Tama every second and get nothing else but Mimitchis and Mametchis.