Apologies, i have raised this question a little too early. I think my hunch was right.
The QAN agent was in waiting state because as PMM logs say , there were no pg_stat_statements view in my RDS and thereby no records to export.
Running create extension as per doc worked. Tested this on test RDS instance.
Doc link - PostgreSQL - Percona Monitoring and Management
CREATE EXTENSION pg_stat_statements SCHEMA public;
Once i created this, the agent went into Running state and i could see the queries. I have raised a AWS support ticket to check if doing this manually affects performnace insights in any way.