''Three cheers for Captain Frodo, Lord of the Ring''

[Inziladun]

Here they are deciding the fate of the free world and Pippin is making light-hearted, if not downright silly, jokes.

Probably so.
Gandalf may have thought that Pippin simply needed reminding of the deadly peril and dark days ahead. Gandalf also knew of the power the Ring had already attained over Frodo, and may have been reacting to the idea of Frodo claiming it for his own (as of course, did happen).