Yes, mass is conserved.
In any chemical reaction mass is neither created, nor destroyed. That is a basic principle that remained unaltered and unchallenged until the discovery of nuclear power. And even then, the basic principle of conservation of mass was simply altered to include the conservation of energy as well. If you with to demonstrate this conservation by the reaction of sodium hydroxide with copper sulfate, you can do so by measuring the mass of all reactants and reaction vessels both before and after the reaction. Just put the beaker with sodium hydroxide solution along with the beaker of copper sulfate solution on the balance scale and measure their total mass. Then pour one solution into the other, and place both the full beaker with the reactants and product as well as the empty beaker back on the balance scale and measure their combined mass. If you perform the measurements accurately, the combined masses both before and after the reaction will be identical.
