Recently had zirconium crowns fitted. I don't like the surface, it isn't smooth. I don't think they were polished properly.

Zirconia is one of the hardest materials we use in dentistry. Because of this adjusting and polishing can be difficult however there are many products made for this exact purpose. Your concerns are legitimate as any surface can attract plaque and rough surfaces are no exception. I would recommend visiting your dentist to have this problem resolved. Hopefully this can be done in the office but if there was an error with lab processing and the entire surfaces of all the crowns are this way they may need to be remade.