Part of your answer can be found in eNotes's discussion of themes for this play, found here.
Disguise and with it deception are used in Much Ado for both good and bad ends. Shakespeare shows us that using disguise and deception to get what you want can be good or bad.
The first half of the play focuses on using deception to help the characters find love. At the masquerade, Don Pedro uses a disguise to woo Hero on behalf of his friend Claudio. When Don Pedro finds out that Claudio is in love with Hero, he says, "I will assume thy part in some disguise / And tell fair Hero I am Claudio." Don Pedro (and all the others) begin Act II scene I masked, and with their masks, the women apparently can't recognize them. Don Pedro takes Hero to dance and flirts with her. Benedick dances with Beatrice and her friend, and Beatrice (pretending not to know it is Benedick) badmouths Benedick to her friend. Every time Beatrice insults Benedick, it is very amusing. Shakespeare's audiences must have been in uproarious laughter during this scene of dramatic irony; knowing that the butt of her jokes was dancing with her would have made her jokes even funnier. Don John is a nasty character who doesn't want anyone else to fall in love or be happy. He pretends to mistake Claudio in his mask for Benedick, and mischievously tells him that Don Pedro is wooing Hero for himself.
Throughout the dance scene, Shakespeare used the masks (disguises) to create all kinds of misconceptions: good (Don Pedro wooing Hero for his friend Claudio), funny (Beatrice badmouthing Benedick right in front of him), and bad (Don John worrying Claudio and upsetting his friendship with Don Pedro.)
Later in the play, Don John uses disguise again to trick Claudio into mistrusting Hero's love for him. Don John's servant, Borachio, makes love to Hero's maid, Margaret, right in Hero's window, and Don John takes Claudio and Don Pedro by the window. Claudio sees Margaret in the window but under Don John's suggestion, mistakes her for Hero. Thus Claudio thinks his fiancée is not a chaste virgin, as he thought, but a whore. In this case, Don John is using disguise for mean-spirited trickery, and the marriage of Claudio and Hero is nearly doomed.
The day is saved when Hero disguises herself to appear dead in order to trick Claudio into realizing and admitting his love for her. So once again, disguise is used for a good ends -- mending the relationship between these two young lovers.