If you want to customize the styles for filter panels in DataGrid from Material UI (MUI), you need to know, where it is. And only so. To update these styles you can only with the object properties.

The example of the solution is here. You need to use slotProps / filterPanel / ....

                  <DataGrid
                    ref={gridRef}
                    ....
                    sx={{
                      padding: 0, margin: 0, fontSize: 13,
                      "& .MuiDataGrid-columnHeaderTitle": {
                        whiteSpace: "normal",
                        lineHeight: "normal", fontSize: 12
                      },
                    }}
....
                    slotProps={{
                      filterPanel: {
                        sx: {
                          "& .MuiDataGrid-filterFormColumnInput": {
                            p: '0px 10px',
                            m: '0px 10px',
                            width: '260px'
                          },

                          '& .MuiDataGrid-filterFormOperatorInput': {
                            width: '160px'
                          }
                        }
                      }
                    }}

....
                 
                  />