First, the interview between Trump and this reporter had occurred something like 15 years before, and Trump, who is interviewed fairly often, didn’t recall this particular interview.
Second, the type of disability the reporter has is one which freezes his hand into one position. If you see a picture of the reporter, the way his hand is is the way his hand is all the time.
Third, Trump used a gesture of waving his hands around to indicate flusteredness. I do the exact same thing. So do many other people who use their hands to talk. This gesture is not at all related to the handicap the reporter has.
One might might mock a disabled person, but the implication is that it is the disability which is mocked. Trump mocked the reporter for being flustered, not for being disabled. The fact that the reporter is disabled is completely unrelated to the mocking Trump did of the reporter.