Throughout Nicholas Nickleby, Dickens establishes as a motif the co-occurrence of undesirable aesthetics and morals in his villainous characters. Ralph, Squeers, and the other antagonist characters are generally described as physically ugly, or in some way unappealing.
Unappealing personality traits will always be associated with unappealing characters (i.e., villains are greedy, misogynistic, insensitive, cruel). Dickens takes this natural characterization choice and expands it to include physical traits society generally considers "undesirable." Many of these physical traits, including obesity or physical "ugliness," are amoral but heavily stigmatized. Neurodiversity, though not necessarily a physical trait, is commonly lumped in with these other "unappealing" qualities, all of which authors tend to uncritically assign to the "evil," morally bankrupt characters.
Multiple potential problems arise from the use of this motif. First and foremost, creating an unequivocal association between "unappealing" physical qualities and "evil" is unnecessarily stigmatizing. While this method of writing may work as a convenient characterization shortcut, it is functionally unempathetic and undermines the writer's own moral standpoint. Dickens wishes to associate Ralph and his villainous cronies with "undesirable" physical traits; unfortunately, readers may find that use of this motif makes any attempt on Dickens's part to garner sympathy for "social outcasts" ring hollow.