The text seems to suggest that this "gem-studded goblet" that is stolen bears no importance in its own right apart from being an object of great value that would be attractive to any thief. The importance of this goblet lies in the impact that it has on the dragon, as the text makes clear:
He had handled and removed
a gem-studded goblet; it gained him nothing,
though with a thief's wiles he had outwitted
the sleeping dragon and driven him to a fury,
as the people of that country would soon discover.
What the successful stealing of the goblet represents therefore is the newly-kindled anger of the dragon, which is something that the Geats will soon discover. It is the way that the thief had successfully "outwitted" the dragon that makes the dragon's ire so terrible, and it is clear that the dragon means to take his revenge on all humans as a result, so great is his anger at having been outwitted. The goblet therefore seems to lack any importance in itself; it is the consequences of taking that goblet that the text indicates are so much more important.