This is one instance of Mrs Malaprop actually making an apt metaphor.

Pineapples were very exotic fruits in 18th century England.

Here is a link to a post I made about the history of the pineapple in England ,during the alst Northanger Abbey Group Read.

Pineapples were used as pinnacles by architects.They appeared on gateposts, carved from stone, on top of buildings, and even in the case of Dunmore Pinapple below , became the whole of a building .

For once she actually made sense in her derangement of words.