For what it's worth, Maemo Mapper does not support reading categories from GPX files with an Import POI operation. In the Import POI dialog, you can only specify a singular category that becomes the "default" category that is displayed in the subsequent POI List. You can change the categories in the POI List, but only after they have been initially assigned the "default" category of your choice.

The best approach, unfortunately, without resorting to external software, would be to divide your POI into separate GPX files, one for each category, and import them into Maemo Mapper one by one, specifying the appropriate category with each import. It's not the most convenient work flow in the world, but it's the best Maemo Mapper can do at the moment.