You don't need the mass murder component in the UN Genocide convention definition. Despite there being actually mass murder when mutants lost their powers and fell out of the sky, drowned or burnt to death
Causing serious bodily or mental harm to members of the group
Imposing measures intended to prevent births within the group
Even if you don't want to count culture which the erasure of 99% of mutants would have had an impact on thats two
"First, the crime of genocide is characterised by the specific intent to destroy in whole or in part a national, ethnic, racial or religious group by killing its members or by other means: causing serious bodily or mental harm to members of the group; deliberately inflicting on the group conditions of life calculated to bring about its physical destruction in whole or in part; imposing measures intended to prevent births within the group; or forcibly transferring children of the group to another group."