This lowers the bar to passing a constitutional amendment. A very bad idea.
It actually would not. All this would do, is nullify any federal bill if 2/3rds of the State Legislatures agree to nullify. It doesn't change the requirement for a Constitutional Amendment.
How so? It itself is a constitutional amendment. But it would empower the states by a supermajority only to veto regular congress-passed laws, not constitutional provisions.