As with all great plays, Twelfth Night has a number of morals that one can discern. Arguably the most significant of these is the importance of staying true to oneself. And yet, on the face of it, just about everyone is engaged in some kind of subterfuge, putting on a disguise to hide their true intentions. Whether it's Viola dressing up as Cesario or Maria writing a phony love letter to Malvolio purporting to be from Olivia, there's so much duplicity and game-playing going on that truth seems to be at a premium.
Even so, the true selves of those involved still rise to the surface. Their disguises of various sorts can only be worn for so long; at some point, they have to reveal themselves for who they really are. Viola may be adept at adopting the false persona of Cesario, but there's no disguising the depths of her true feelings for Duke Orsino.
By the same token, the machinations of Sir Toby Belch and Maria have shown that, with regards to the hapless Malvolio, there was always a fool beneath that stiff and superior countenance of his. Malvolio always thought that he was better than anyone else, but Sir Toby and Maria have demonstrated conclusively that when it comes to matters of the heart, he's as foolish as everyone else.
Further Reading