Actually no. A liar is one who lies. That is objective. The only degree is relative to others who lie. Compared to others, Trump is a massive liar. He has objectively been shown to literally lie about anything, even crap that doesn't matter.
What truly distinguishes him is that most other people, when confronted with their lies, either apologize or make an excuse for their lie. Trump, in contrast, never apologizes. To the contrary, he usually doubles down when confronted with facts. Occasionally, he denies he ever said the lie, even when he has been recorded doing so.
This is why his lawyer Daud [sp] advised him to never, under any circumstances, sit down for an interview with Mueller, lest he wind up in prison.
My personal favorite Trump lie:
"I own one of the largest wineries in the country." That's ****ing adorable.