Double click in the bill of material and scroll over to the mass or weight column. Right click on the column and select column format. Click apply units format and make sure display units type is unclicked. You can also rename the column to something more normal like "WEIGHT" and edit the units display so that precision is a little more realistic than three decimal place default.
Once you have the units displaying to your liking you can save this to a sheet format so you can reuse this BOM convention for your next drawing.
