Hi I was just wondering  if I had a character  who could use  true polymorph  into a storm giant and fall from 60ft onto a baddies lets say a ancient black dragon head how much damage would it do considering they weigh  15,000 pounds 

6d6.

1d6 per 10’ of fall. Mass of the object does not matter, by RAW.

Technically the falling creature takes 1d6 damage/10 feet (max 20d6) if they land on anything that isn’t another creature

The optional rule in Tashas’s is that if a falling creature lands on a creature and neither is Tiny,  the creature that gets landed on has to make a DC 15 Dex save to jump out the way or they split the damage even (rounding town) and they both go [Tooltip Not Found].

You need to consider that D&D has two halves - game and simulation. If you go full simulation, you break the game and vice versa. Dropping huge things on enemies is not well represented in D&D because it would quickly become better than most other actions. Every encounter would involve the party luring the enemies over to the nearest cliff or unstable column or whatever and dropping a boulder on them.

If you wanted to do this one time for a lot of damage, many DMs would probably be okay with it. But if you're doing 50d10 by dropping a Sperm Whale on stuff with a 4th level spell, why wouldn't you use that all the time? It breaks the game.