A nuke with yield of 64 megatons is not enough to strip the atmosphere off a planet,

Some quick google searching shows that this isn't that impressive. We're only talking about 1.5 gigatons per torpedo. Even then, this is an incredibly-high-end yield, flying directly in the face of other sources which pin them at 64 megatons on a good day. You can't just say that trek tech can do more damage than it can because star trek tech is better. Also, if photons could preform such a feat so easily, then how come you need more than 1 torpedo for orbital bombardment? Just shoot one and everything on the planet is dead.

Also, the sort of power a turbolaser broadside puts out would easily put out the same or greater results. We're talking 10 or so of the asteroid impact that killed the dinosaurs every second.

can hit a target at 10 light minutes....

When has the Enterprise managed to track and hit a ship from 10 light-minutes away? It's one thing to do the math; it's another to track the ship and hit it in an acceptable time-span.

And since asteroids are usually made up of softly packed dirt it makes sense that you'd be able to just blast it apart

You can see from the film that the asteroid is completely vaporized. Also, this wasn't even the main guns that did it; it was a much weaker point-defense emplacement.

but when a sand-ball hits the star destroyer the star destroyer tends to come out worse for it

The gunners don't even bother shooting asteroids that have roughly the same impact as a photon torpedo.

We also don't really see the ISD's weapons creating 200 gigaton blasts

We do when they need to boil a planet's worth of oceans and atomize the crust. We also do when we compare the times of battles to the frankly ludicrous shield outputs.