I don't get this. Does cladistics dispute the "hair, mammaries, three-ear-bone" characterization of mammals?
No such thing as "reptiles" makes sense - crocodilians and birds are both descendants of archosaurs, but lizards aren't, and mammals branched off somewhere in between.
Those characters distinguish a clade within reptiles. As you note mammals branched off after some, but before most, major reptile groups. So, on the cladistic approach, "mammals" can't be a group with equal rank to "reptiles". Although this is the way we classify them. If we were consistently cladistic "mammals" would have to be a subgroup of "reptiles".