Is it possible to create RDS replica in public subnet while master db is in private?

As long as subnets are part of the same DB Subnet group all can be done

see documentation below of creating read replicas[1]https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_ReadRepl.html

Ideally you would have a private subnet for primary RDS instance, hence the replica would use the same subnet group and would not be accessible publicly. You cannot place the replica in a different subnet group. If your subnets are public, but primary DB has Publicly Accessibility set to No to ensure it can only connect privately, then you can have a public replica with Public Accessibility as Yes. I wouldn't recommend making the subnets public here. Here are some approaches:

1. Setup manual replication to an RDS instance which will be public, and use that for reading purposes
2. Use AWS DMS to move data and changes similar to 2 instead of manual replication setup
3. Use cross region replica(might not fit the use case) within a subnet group which has public subnets in another region