Obama was mocking himself, Trump was mocking a disabled person by mimicking his interpretation of the person's real disability. At most Obama's incident may be referred to as an insensitive gaffe. Trump's incident was a full blown dirtbag lowlife attempt at bullying.Obama did make a joke referencing the special olympics.
LINK?
Here is a search, how about that?
obama special olympic joke - Yahoo Search Results Yahoo Search Results
Watched the video in question and it was nowhere near as offensive as the muck that drips out of Trump's face ...