The only way I know how to do this is with a table calc that you put on the Filter shelf:
IF INDEX()/SIZE() <= .2 THEN "Show" ELSE "Hide" END
The default partitioning of Table (Across) will cause this calculation to order the data by whatever your sort order is, and take the top 20% of those rows. It's important to note that filters on table calculations are performed after most all other calculations, that can change your results.
Sample workbook attached.
show top 20 percent.twbx.zip 1.1 MB