Answer: It reinforces the idea that Bertie is not happy about Aunt Agatha's visit.
Explanation: In Chapter 2 of "The Man with Two Left Feet" by P. G. Wodehouse, Bertie is visibly annoyed with Aunt Agatha's visit. He already hinted he was not in the mood when he stated "I was feeling like a badly wrapped brown-paper parcel. I'm never at my best in the early morning. I said so."
On top of him not being a morning person, and dealing with an unwanted visitor, he was also hungry. He is expressing to both Aunt Bertie (and the reader) that is not in the mood.