Is there a way to view WAL updates on a Hot Standby server?
pg_stat_statements has nothing to do with WAL. The WAL doesn't record statements and you cannot discover what statements were run from WAL. It only records blocks changed on disk and relevant related metadata.

It's definitely not useful on a standby.

You could use pg_xlogdump (pg_waldump on pg10 and newer) to view the low level WAL stream, but without a lot of experience and time you won't be able to understand it. It doesn't and can't show table names and row info, only table relfilenodes and tuple TIDs, plus binary tuple data.

If you want to avoid installing pg_stat_statements you could make a poor-man's version by capturing pg_stat_activity on a timer and doing your own parsing and merging.