What is Mercutio saying about Romeo in lines 14-18 of Act 2 Scene 4?

Mercutio says Romeo can change his appearance but he will not be able to change who he is. He is saying this because Romeo has been pretending to be someone he is not by disguising himself in order to attend the Capulet ball, but Mercutio knows that he will still be the same person under the disguise, and that the changes on the outside will not really amount to anything.