I'd also like to mention that the Malfoy family is affluent. Everyone would know about this party, I believe, because everyone would want to marry Amelia (she's rich...).

Pettigrew's true identity is still hidden. He's still, on the surface, the best friend of the Potters. It would be highly suspicious if he were to show up at the Malfoys' party. It would probably blow his cover.

Second, Pettigrew is someone Malfoy looks down on. He's snivelly, and weak, and whiny, and not-so-affluent. I doubt Malfoy would want to be associated with Pettigrew, someone who once had close relations with the Potters.

Finally, I can see Pettigrew wanting to go to this party. He likes to surround himself with powerful people. Of course he'd want to be at a party with many rich, powerful, and well-known people!

He may not be the most social man, but he does try to win the favor of those he thinks are the strongest in the game. This is conducive to him wanting to go to the party, but I don't think the Malfoys would want to invite him AND it would blow his cover as a spy.