Brown Betty is a dessert, like a crumble. It doesn't get a good review in the novel, as teens generally look at food as something that get in the way of other exploits, like girls. Holden cannot let himself look forward to anything, no matter what.
It could be said, however, that because Holden is an underachiever, nothing could raise his spirits, even sugar. This could be said to be a precursor to the generation of today, where teens revel in not being excited about any prospect of or for the future.