Well, I don't dispute the similarity between the mouse's and Hilda Baker's comments, but surely it can't be necessary to find something funny today that one be aware of some throwaway line in a British TV programme from about 30-odd years ago!
I found it amusing just as it is, myself.
The surrealistic element is similar to the sort of thing one finds in Eddie Izzard's comedy all the time. For example, he describes how his cats often disappear behind the settee and purr. It would probably never have occurred to most of us but what he thought was, "I wonder what they're drilling for?" Only when we hear that does it occur to us that cats' purring DOES sound like a distant pneumatic drill.
Same with the mouse and the elephant. Of course the elephant would see the mouse as small at any time, but the mouse sees himself as small only now that he's not very well. Before that, he was quite big in his own eyes.