I agree it's an unusual combination, and I probably would not have come up with it on my own. Here's how it came about:
On Saturday of the Buffalo-Rochester Roadfood tour, several of us went to Antoinette's after the official tour was over. I was looking through their list of sundae options, and I saw "mint-pineapple" on the menu. It caught my eye as unusual, but I chose another unfamiliar topping, the butter-rum.
On Sunday, the tour went to Parkside Candy, and I saw mint-pineapple among the sundae toppings there as well. I interpreted this to mean that not only did some intrepid Buffalonian think this was a good idea, someone else thought it was a good enough idea to copy it. But the hot fudge sundae had been strongly recommended at Parkside Candy, so we had that instead.
On Sunday evening, we went to Antoinette's again, and I was curious about the mint-pineapple, so I talked Lori into giving it a try. Since "mint-pineapple" was hyphenated, I was vaguely expecting a single topping with both mint and pineapple flavors, but it turned out to be two separate toppings. (I think the layering was ice cream, pineapple, some mint syrup, whipped cream, more mint syrup.) I wasn't planning the peanuts specifically, but they came with the sundae.
The combination of flavors turned out to be delicious. The mint turned out to be a splendid intermediary between the acid sweetness of the pineapple and the creamy sweetness of the ice cream. The roasted peanuts added delicious crunch.
This was one of my favorites in a weekend full of Roadfood standouts. I'm not sure I would drive to Buffalo just for this sundae, but I would definitely order it again if I were in Buffalo, and I'm likely to pick up some mint syrup and pineapple with other sundae supplies next summer.
The short form of this story: you can get that combination of pineapple, mint, and peanuts by pointing at one line on the menu and saying "I'd like that, please."