In a liberal democracy, it is the responsibility of the opposition to challenge the party in power. It is the responsibility of the President to rise above pettiness, to unify, to lead, and to embody the values of the citizenry.
This is the definition of petty, and by responding to it, Trump demonstrates smallness of character and failure to lead by example. The fact that he chooses to engage on minor and perceived insults, but has chosen to make no effort to unify our country or stand up against the hateful acts that are occurring everyday in his name, shows just exactly how meek a man Trump is.
If it wasn’t for the wealth he was born with, his character never would have allowed him to be anything but a two-bit con man. Which is exactly what history will reveal him to have been, once all is said and done.