In Much Ado About Nothing, according to the stage directions for the dance, Don John is not masked during the revels. Why?Act II

This is a very shrewd question. Well done for noticing
this tiny detail! If you think about the context of the rest of the scene in Act II
scene 1, you can see that it is vital that the major actors are masked: only with this
disguise can the merriment of tricking Benedick and Beatrice go on. However, consider
what Don John is planning to do. It is vital that he is not mistaken for anybody else.
When he talks with Claudio it is important that Claudio knows that Don John is Don
Pedro's brother, thus guaranteeing the veracity of the scurrilous lie that Don John and
Borachio tell him regarding Don Pedro's intention of marrying
Hero.

However, aside from this, I think there is a further
distinction we can identify. There is an irony in that Don John, whilst not pretending
to be anyone else, symbolised by the mask, still has a "mask" on his face, but one of
deceit and treachery. This mask, unlike the others, will only be taken off towards the
end of the play. Therefore having him "unmasked" draws attention to this metaphorical
mask that hides his true intentions from the other characters.