I think it will work, but the problem was not that it wasn't handling IfcMappedItems - the problem was it was using unresolved representation to get representation items (e.g. #245401=IfcShapeRepresentation(#27,'Body','MappedRepresentation',(#245400)) isntead of #8165=IfcShapeRepresentation(#27,'Body','SweptSolid',(#8163))). Representation can be resolved using ifcopenshell.util.representation.resolve_representation.

Haven't seen bim.unassign_representation_item_style implementation before, but it feels odd - it does one thing for the active object (unassigns style for active representation item specifically) and something very different for all other selected - unassigns style from all their representation items.

It feels like batching option should be available on shift+click or something, that would also allow unassigning current style from all representation items of the active object too, to make operator's effect consistent.

Another thing noticed - it's also kind of odd that it currently unassigns the style from all available representations of the element (it's iterating over all element.Representation.Representations), not from just the active one.

Thanks @Andrej730
These comments make sense to me.
I'll see if i can dig into it more. :)