Status of non-top mutated creature cards
The cards are considered only 1 creature; 1 permanent. The cards on the bottom do not exist in any meaningful sense as their own thing. They are not objects in the game; so nothing could target them. A bottom card has no types, no name, no mana cost, nothing; so it would not interact in any way with the legend rule.

From the Ikoria mechanics article:


  As a mutating creature spell resolves, instead of entering the battlefield, it merges with its target into one creature.


Emphasis mine. The new spell with mutate, if cast for its mutate cost, never actually enters the battlefield.


  You'll end up with a pile of cards (or tokens—we'll get to that in a second) that all represents a single creature. That creature will have all the characteristics of the top card, and it will also have the abilities of every card underneath.


So there is only a single creature. The only text on the non-top card that has any meaning is the abilities that it grants to this single creature.