Index fillfactor and index pages in PostgreSQL
Table and index fillfactor are independent from each other.
Both tables and indexes are stored in files on the operating system. These files are organized in 8KB blocks. fillfactor determines to what extent these blocks are filled by an INSERT (in the case of a table) or the initial creation or extension on the right end (for an index).
Even though the fillfactors on table and index are independent, the indication for setting lower values is similar for tables and indexes: if the table receives many updates. Bear in mind that many table modifications also cause an index modification.
Note that a lower fillfactor on the table will only help if none of the updated columns are indexed. Otherwise, it is only a waste of space.
I would set a lower fillfactor on tables if the above condition is satisfied and the table receives many updates. I would not set a lower fillfactor on indexes unless I see that an index regularly gets fragmented by many updates.