Don
At those sorts of near-light speeds, you'd get total mass-energy conversion. It would be like a nuke going off as it hit. Make a slug of
something dense (like diamond or depleted uranium) with hyperspeed engines and a navigation system, you'd have a missile that could jump to a system, then take out almost anything.
That's why this sort of trick looks spectacular in the movie, but when you think about it, it breaks so many things. How many ships or cruisers could have been saved by ramming one of them into a super star destroyer, or a death star at light speed? TLJ has made a fundamental change in the rules of the movies. It's like suddenly turning up with modern guns where everyone else has swords.
You know destroying your own ships to take out enemy ships is usually the desperate last acts of a military?