It wasn't the rubbles though. It was the ground or, as the citizens said, it was the entire city.
Don't get me wrong, you could have a point in saying it's not as impressive as it sounds, but here you're just trying to low-ball it by making stuff up.
You could have said it was because the ground was shallow as it had the toy factory beneath it, but you didn't say that, you said these were all just rubbles when we clearly see the city, along with a crazy amount of buildings, getting folded like a sandwich.
And even if you wanted to argue that it's not as impressive as it looks, let me remind you that Elizabello literally tried to use his King Punch to get out of the toy factory by punching the wall in order to create a tunnel, and yet it didn't even create a bump, to which one of the other gladiators said : "It's normal. It's not a wall, you're literally punching the ground."
And yet Luffy literally destroyed what Elizabello couldn't even scratch by sending Doflamingo flying into the ground.
However, the biggest feat in all that was actually Luffy destroying all of Doflamingo's haki-coated awakened strings in one punch when Law himself (who cut a literal mountain) couldn't even cut some small regular strings nor go past Doflamingo haki (who only got a slight paper cut by grabbing Law's blade in the middle of an attack).