Yes it is perfectly OK to put insulation over floorboards. The main disadvantage is in not having the flat space to store stuff up there anymore.
The job is very easy to do, but extremely unpleasant because of the fibres. Buy a nose and mouth dust mask, wear old clothes, and have a way of removing clothes into a bin bag for future washing. Wear thin gloves and cover a such of your skin as possible, the fibres may itch but not hazardous or carcinogenic.
It is very cheap job to DIY, which is made expensive by tradesmen as it is unpleasant.
The only other thing to warn against is not to block the eaves ventilation grilles, if you have them, and don't insulate under the cold water tank, if you have one.