For mobility scooters that need to display a tax disc you can buy tax disc holders that can be attacted to the front of the steering area, so that it is prominently displayed.
Some mobility scooters are not for road use and dont require any tax disc to be displayed but for those that do its best to keep it visible, having it displayed on the front is probably the best area to display it.
It’s only the Class 3 mobility scooters that require this as they are legally allowed to be on the road and can have a maximum speed of up to 8mph.
This type is registered with the DVLA because of the road use and the tax disc displayed is a nil duty tax disc and to register the actual mobility scooter and have it licenced a form needs to be completed and sent to the DVLA.
You may find the information provided at the following link useful Code of practice for class 3 vehicle users it provides answers to questions that you may have regarding the use of powered mobility vehicles on the road.