Tiling on plasterboard in bathroom

Yes you can tile direct onto plasterboard which is a better surface, the board can hold in weight 28 kg per m2 where as skimmed only 20 kg per m2, but if the board is also in a shower area it must be tanked to make it waterproof. To the further comment made, no plasterboard is waterproof, the green board is only water resistant, normal board can be used but not in wet areas which will have to be tanked including the green board, and the weight does matter as there is a limit set out by BS. And on this question, it is better to tile direct than to skim it first! I would advise that a bathroom is to be boarded with tile backerboard(which is as well only water resistant)but will not degrade with moisture ingress, but this was not the question asked.

Yes you can

If you did your plasterboard with green plasterboard which is waterproof, then it's okay but regular one it's not recommended! It does not matter if that plasterboard holds weight or not! if regular plasterboard which is white gets in contact with water or moisture then it can cause tiles to come off! Hope this helped you!