You can try PivotTable's map property to modify both the yellow (row header) and red (data) parts:

https://www.koolreport.com/docs/pivot/pivottable_and_pivotmatrix/#properties-map-(version-%3E=-5.0.0)

For the row header map, you can check $headerInfo parameter to see its field name to decide whether to format its to numeric type:

\koolreport\pivot\widgets\PivotMatrix::create(array(
    ...
    'map' => array(
        'rowHeader' => function($rowHeader, $headerInfo) {
            print_r($headerInfo); // for the first time run, print out $headerInfo to see its data structure and value
            return $v;
        }, 

For the data map, you can check if its numeric value is negative to convert.