On a cold day I'd like to wear a sweater doesn't have moth holes. Why would you use packing peanuts which cannot be at all densely packed? It would probably be less effective than crumbling newspaper and throwing that around the attic.

If you covered the peanuts with material that stopped air flow you might get some benefit, but that is pure folly.

Bring the packing peanuts to your nearest Mailboxes Etc. and get some cellulose blown in that attic instead.