Fun fact. If you shot a marshmallow at the Earth going at 99.99% the speed of light, it would hit with the force of about two thousand Hiroshimas, if I've done my math right. If you add a few more nines to the percentage, that power output goes up exponentially. Let's say that it wasn't at light spee So imagine how much more powerful a lightspeed kamikaze run for a ship normally weighing one ton would do, especially when they're going faster than light speed. If you had a big ship, it would destroy planets with the intensity of a billion stars.

So...people have brought up why this kamikaze thing hasn't happened before, and I've thought through reasons why it might not have. First, there's obviously a flaw with the whole Death Star thing, because if people can blow up planets just by launching a lightspeed nyan-cat their way, making a giant station to do that it a bit redundant. In other battles, though, I can see it not being too advantageous. Warfare is generally fought over resources and political gain, and you can't always reach your objectives by blowing everything up. So a lot of the ships seem to be designed for troop transport. You also might not want to kamikaze a ship like the Death Star if it's nearby an inhabited planet, or even in the same system as one, because it shine with the brilliance of a billion trillion suns if you hit it at lightspeed and the radiation would kill everyone. Even when the target in question isn't nearby an inhabited planet, is your lightspeed kamikaze run really going to be accurate enough to hit a moving target that's lightyears away?

So it kind of makes sense why we haven't seen this before. When Holdo rammed the enemy fleet, there weren't any inhabited planets nearby. There's still the technicality of how the Rebels/Resistance didn't get hit by the shockwave, but my speculation is that she didn't quite reach light speed and was just accelerating when she hit Snoke's cruiser. Looking back, the only other situation that I can think of where someone was in a strategic position to use lightspeed kamikaze tactics was when the Empire wanted to take out Hoth, but I think that they wanted prisoners and, if I remember correctly, Darth Vader discovered that Luke was his son before he found his whereabouts on Hoth.

EDIT: And before people say that Han Solo could have done this to Starkiller base, he probably didn't want to do that while his son was on it. Of course, the Resistance could have launched a hyperspace missile that way without asking him...

Darn, I guess that it is a plot hole..