Value is a default presentation mode of Fiori Overview Page pie chart. If you want to know how to switch between value and % of total then this blog is for you.
Below is pie chart default presentation mode.
Using plot area data label chart property it can changed to % of total.
This is how plot area data label chart property can be manipulated in manifest.json
For more Fiori Elements tips and tricks please refer to my book SAP S/4HANA Analytical Applications with Fiori Elements