[cf. 1922, 5:26-30; Gabler 1:94-99]
Sadly, this is probably not what Mulligan has in mind. Rather, Mulligan means that he’s disguised, he’s pretending to be something he’s not.
The tradition of mumming came to Philadelphia from many places, but the strongest thread runs from Ireland & the other Celtic countries. By tradition, around the holidays, a gang of costumed men would go from house to house and basically trick or treat for booze. There might be a play or a performance involved, but there’s a costume and some kind of entertainment and probably “something sinister” in having them come into your home… as Mulligan suggests.
And as for sinister… here’s another question for the masses. Was Joyce left-handed? Stephen, based on a number of references in this book, seems to be a leftie. And Joyce’s corresponding figure in Finnegans Wake, Shem the Penman, is left-handed. Of course, even if Joyce were left-inclined, no school in Ireland would have let him actually write that way…