Speaking purely of the play in itself, it's usually taken that a character does not lie in soliloquy. If he said that he is "determined to prove a villain" because he is "deformed", then that is taken as simply truth. Or at least, that he believes it to be the truth. While there are some exceptions and caveats to the rule, any interpretation of the play that takes a character as lying directly to the audience must have an extraordinary justification. After all, the whole point of a soliloquy is that there is nobody else around to here them who can affect the action, and thus no reason to lie.
That said, it's worth looking closely at the text here, since the question asks if he is "disabled". He is not, in fact, said to be disabled. What he says primarily in this speech is that he is ugly. He will dwell on it while looking at at a mirror, and will not be suited for seduction, but he suffers no loss of function.
Only a single word hints at any kind of handicap: "halt", which in this context means "limp". Even so, there's no indication that he cannot walk, or even that he requires a cane to walk with. Perhaps he has the handicap of not being able to run.
In Henry VI part 3, Richard slays Clifford, in revenge for the death of Richard's brother Rutledge. That play, while not formally part of Richard III, may be be assumed to be in the minds of Shakespeare's contemporary audience. Clifford was himself a formidable adversary. Whatever disabilities Richard had, they did not prevent him from being a serious warrior.
This is in conflict with other things said about Richard elsewhere in the text of Richard III, which do include his hunch back and withered arm. These present serious questions for a director: unlike this text, they are not said in soliloquy, and should not be taken at face value. The withered arm, in particular, is made in the context of a false accusation, and may simply be a lie.
See how I am bewitch'd; behold mine arm
Is, like a blasted sapling,
And this is Edward's wife, that monstrous witch,
Consorted with that harlot strumpet Shore,
That by their witchcraft
thus have marked me.
Here, he says that the withered arm is a recent occurrence: it's not something he was born with, and for all we know it may simply be a lie. An absurd one, if false, but hardly the most audacious of Richard's lies. Lying is his great gift.
So we may take it as truth that Richard sees his ugliness as a key motivator for his action in the play. But that ugliness itself is not disability.