maybe, but according to the same Constitution that creates the existing system, this should be perfectly legal. Ultimately each state gets to send Electors for the President on its own terms.
However: several Supreme Court justices have signed on to the “independent state legislature” theory, a radical and unsupported interpretation which holds that state legislatures can’t be restricted in how they pick electors (even by their own laws and constitution.) I assume that if states actually adopted the NPV act, the court would instantly bypass it. https://en.m.wikipedia.org/wiki/Independent_state_legislatur...