Mr X Billups, who makes just this one brief appearance in the novel, may not appear to have much significance in himself - apart from to provide some slight comic relief, perhaps. However, as part of the colourful gallery of characters that the children see coming to attend Tom Robinson's trial, he serves as a reminder that there are all types of people to be found in every society. A central lesson of the book is to recognise that many kinds of individuals exist and that it is important to accept them for what they are. This is what the children have to learn, despite the deeply ingrained prejudices that are upheld by seemingly respectable members of society (like their own Aunt Alexandra) who discriminate against others on the basis of colour, background, creed, and gender.
Mr X Billups is only one of the odder characters that the children see milling around in front of the courthouse; there are others, like Jake Slade who, according to Jem is 'cutting his third set of teeth'. There is also Mr Dolphus Raymond who is supposedly perpetually drunk and lives with a black woman with whom he has several children. This, from the general viewpoint of Maycomb society, makes him very peculiar indeed - irredeemably so. However, Scout and Dill discover that his drinking is only a facade:
Secretly, Miss Finch, I’m not much of a drinker, but you see they could never, never understand that I live like I do because that’s the way I want to live. (chapter 20)
'They' refers to the greater, and racist, part of Maycomb society who simply cannot comprehend how a white man could live quite openly with a black woman and have children with her. Mr Raymond blackens his reputation in their eyes even more by pretending to be drunk, but, he says, at least that gives them some sort of explanation as to his conduct.