Losing every swing state and failing to even win the consolation trophy of the popular vote after even Hillary fucking Clinton managed that much is something I'd call getting your ass handed to you. The US election system is terrible, but it's the game they were playing and Trump won hands down.
Also, not that it's the point but I have to note that technically most election victories are decisive, in the sense that they resolve the winner with little to no ambiguity (which is usually the case, even when the margin is narrow). In that sense, the only way Trump's victory is not decisive is if you contest the legitimacy of the whole election.
Yea I know, had to get that digression out of my system, especially since you said "in any sensible meaning of the word" and all. Sorry, I didn't mean to nuh-uh you on semantics, just point out something that tickled my pedantry sense.
Edit: I also brought it up because IMO "decisive" is a bit of an odd choice to describe election victory, unless referring to some grander context where the election marks a major historical turning point in national or international politics in favor of the winning side.