You may think this is a simple question more to do with obstacles than grappling hooks.

If I "park" my droid on a small asteroid and the base of the ship overhangs the asteroid token does it gain a defence dice for being obstructed by the asteroid?

Only if the shot is obstructed by the rock. You are ignoring its effects, your opponent is not.

hmmm kinda guessed this - I need bigger asteroids

Or you need to land on the 'far' side of the rock, which will obscure you from that direction, even if not others.

But, yes, if using grappling claws, the bigger the rocks and debris you can use, the better.

They are not hooks nor are they claws they are struts. GRAPPLING STRUTS.