There are a few potential complications with using relativistic dropships or fighters.
One deceptively big one is aiming. While you can calculate vectors with extreme precision even using pen, paper, and calculator, getting a ship in the vacuum of space to be pointed in exactly the right direction when basically you change position by it shoving itself in different directions with a rocket is a lot more complicated. Over the course of months of burning, even tiny errors in course can become huge. And things moving at a significant fraction of c have much less time to correct for it. Depending on how significant that C fraction is, maybe a WHOLE LOT less because of relativistic affects. So accuracy may be a question.
Space dust is also probably a problem. Maybe a small one, but the longer you take to run your projectile up, the higher that risk becomes and the more potentially catastrophic it becomes to your projectile.
Also, you have to commit to this sort of thing well in advance. We're talking a lead time of months to a year before your shot lands. It's not very practical in a fluid geo-political situation when you can accomplish most of the same thing with a bunch of nukes with less time.
Also, cost. With all those space rocks floating around for free, there's gotta be a cheaper way of doing this sort of thing.
It's kinda the grandest of grand gestures. Even the most super-villainous of super villain factions doesn't really have much of a reason to C-fraction a ship into people when there are more efficient means of mass destruction.
Though I will point out that the Word of Blake loved to throw space rocks at people, and the Taurians did use dropships as kinetic impactors during the Jihad. So it's not strictly unprecedented.