QGIS cannot read the valid-pixel expressions or masks which belong to a BEAM DIMAP product, they are only used in SNAP. For QGIS the raster is simply a matrix with values. GeoTiff also doesn’t store such metadata.
If you want to permanently remove these values, you have to replace the raster values, for example in the band maths.
IF band_1 == mask THEN 0 ELSE band_1
This replaces any pixel within the mask by zero . If you then open the raster in QGIS you can set 0 as the NoData value (in the transparency tab): S1 tile in QGIS: Zero values