I do not think you can configure it
That portlet uses the Team Allocation as the data provider. That has some timescaled fields that it time slices (system slices) and the options for displaying them are
No percentage option. You could create a new virtual time scaled field, but the display options are the same.
If you want percentages then you have to manually create the segments in the resource properties page. Or use a process.
You could also have a custom query based portlet where the allocations are on monthly columns and in percent, but query based portlets are not editable.