There is a broad interpretation possible for so short a quote.
Banquo is describing the witches as he and Macbeth happen upon them. The full quote:
"What are these, so wither'd and so wild in their attire, that look not like the inhabitants o' the earth, and yet are on't?"
implies that Banquo is so struck by their appearance that he suspects they are not human. By "withered" and "wild" we might presume that Banquo assumes humans, or perhaps women in particular, to be better looking and better kempt; we might even go so far as to assume Banquo believes there is a limit to human ugliness and behavior, and that the witches exceed it.
The audience's impression of Banquo from this might depend largely upon the identity of that audience; an audience composed of feminist academics would interpret this very differently from a group of 16th-century English farm workers. We might think that Banquo is a chauvanist who has committed a capital sin by placing his own conceptions of beauty in conjunction with what he considers to be human, perhaps leading him to interpret the "Other" as nonhuman and therefore not worthy of human rights. On the other hand, Shakespeare needed it to be clear that the witches were not "earthly" beings, and therefore not part of what we would consider the natural or reasonable world; it is common in literature for appearances to be linked to motivations or thematic points in a story, and the witches' unusual appearance, i.e. being "inhumanly ugly" is meant to strike the audience with a sense of unease and mistrust. Indeed Banquo goes on to be a voice of reason to Macbeth, and it is he who sees through the seductive veil of the witches' prophecies to the dark forces they represent; we might argue that he knew this from the start, as evidenced by the quote in question.