I don’t see why it shouldn’t be. Of course it’s not of vegetable origin, but so what? It’s added in small amounts to season food - that falls within the broad definition of spice. It might not be pungent or aromatic, but not all vegetable origin spices are either.
I tried the PB + black pepper thing - it was interesting - the peanut butter seemed to bring out the earthy, woody tones in the pepper. It was quite good.
Peanut butter+strawberry jam+chilli sauce is quite nice.
It doesn’t involve J, but peanut butter, mayo, and lettuce is actually a really tasty sandwich. I rarely have lettuce on hand, so occasionally I have just PB and mayo when there’s nothing better to use the bread for. Tastes okay, but lettuce really helps.
I had forgotten that PB and lettuce was one of my mom’s favorites. I don’t recall any mayo, though.
My mom likes mustard on pancakes. I’ve tried it and it’s actually quite good. Makes it into a savory dish. Pepper would just make it that much better. No peanut butter was involved tho…