Here is the reason why it is currently not possible to create a fully custom DR tracker (for example with adjustable icons, size, position, etc.).
In the new patch, Blizzard introduced a new event that fires in arenas whenever a diminishing return is applied or refreshed:
UNIT_SPELL_DIMINISH_CATEGORY_STATE_UPDATED
The problem is that this event returns secret values. Because of that, addon authors cannot safely use the information provided by the event for any programmatic logic (such as mapping categories to icons, tracking states, or updating custom UI elements).
I have already reported this in-game, because it is hard to believe this restriction is intentional. From an addon perspective, allowing arena players to create their own custom DR tracker would be a very small change on the game side.
This feels like something that could be fixed with a simple hotfix.
I hope this issue gets some attention and can be resolved soon.
Best regards