Was it valid to say that King George's goal was the "establishment of an absolute tyranny over the American colonies?"
No, it was not valid from an objective point of view to say that the King was trying to establish a tyranny over the colonies. However, he was trying to reduce their rights in ways that some in the colonies would have thought were tyrannical. I would compare it to the present situation with President Obama. Is he really trying to bring socialism to the US? No. But he is doing things that you can call socialist if you already don't like him.
Colonial elites like those who wrote the Declaration wanted to be independent and were going to use hyperbole and anything else they could to advance their cause. Therefore, they took things that the King did that were not really tyrannical and inflated them.
For example, they took the Coercive Acts (which were punishment, after all, for the colonists' destruction of a huge amount of property at the Boston Tea Party) and argued that this was an example of the King trying to take away the right to self-government. Or they took the Quebec Act, meant to help assimilate the newly-taken territories in Canada, and argued that it was a plot to favor Catholic French people over Protestant colonists.
So, King George did some things that did take away from the colonists' rights, but it is overdoing it to say that he was trying to establish a tyranny.