Do we need a term to describe someone who regularly says things that are untrue?

Liar is too strong a term. To be a liar, you need to know that in fact what you are saying is untrue. It could be a matter of ignorance. In Trump’s case, I believe it is a matter of complete indifference to the truth, which is not quite the same as lying. He just makes things up on the spot. It would then be incorrect for media to say Trump is lying, but it would be appropriate to point out that he frequently says things that are, at the very least, highly inaccurate.