That Kartoffelpuffer looks like a Spanish tortilla... As my grandparents were preparing many german dishes, that combo of salty potato with applesauce didn't look too weird... 🙂
On the other side, I don't like to mix sweet and salty in the same time... 😃
Spanish tortilla
I make a good tortilla, too, myself. But this is a very different dish:
It's from raw potato and onion (minimal egg) - the potato starch from shredding the potato binds the patties.
I would agree about mixing savoury and sweet stuff, normally, but there are exceptions: berrys/jam on game, berrys/jam and certain cheeses (camenbert) and... Kartoffelpuffer mit Apfelmus.
Don't knock it until you've tried.