For me I am not sure public mooring buoys are there for you to reserve in this way.
Try it from the other side......How would you feel if you arrived with a need to tie up, and found all the places all 'reserved' with dinghies. Would you frown upon what the others had done or found it OK?
To me I find this a bit like the Germananic trait of creeping out early hours to cover sunbeads around the hotel
pool so as to 'reserve' space for after breakfast.
Nothing really wrong with it provided no one gets upset if you remove the towel / or tie up to the dinghy
as the spot is clearly not being used.........but one can argue the case when you (or they) return.
But IMHO the mooring buoys are not there not to be used - and to try and stop other sailors using them when you are not, is not good 'form'. In my view of course.