Under absolutely no plausible assessment of the evidence is there an emergency at the southern border, and even if there was, no evidence exists that a wall would solve the problem.
But more importantly, this is a complete abuse of power. The president asked Congress for something, Congress had plenty of time to act and decided not to give the president what he wanted, and then the president did it anyway. The National Emergencies Act is not supposed to be a way for the President to ignore or override Congress and rule by imperial fiat. There must be checks and balances to preserve our form of limited government. If this emergency declaration stands, then Congress is essentially relegated to an advisory or symbolic role as presidents can simply declare a national emergency to do whatever the hell they want—and we no longer live in a democratic republic.