Maybe in the Trump case, since he had fewer than half of the people that bothered to vote, but Clinton won the majority of those who bothered to vote in the primary, so even if you think the choice of her as unrepresentative of those who could have voted in the primary, first past the post isn't really the culprit, political apathy is.

That said, I generally agree that first past the post is bad and should be gotten rid of.