Would a person who has no assets and whose only income is from Social Security Disability be considered judgment proof?

It is very likely given the circumstances that the person would be "judgment proof". The legal term is actual execution proof, meaning that a creditor could sue and be awarded a judgment, but the judgment could not be enforced. The vast majority of person's who are sued are execution proof. However this does not stop creditors from filing a lawsuit in the expectation that the debtor will eventually have property that is not exempt and therefore can be attached.