What are the main monitoring or other parameters to check if the instance is underutilized—the MySQL RDS instance?

Hello.

The CloudWatch metrics I often see for RDS are performance-related metrics such as "ReadIOPS", "ReadLatency", and "ReadThroughput".
By looking at these metrics, you will be able to understand whether loading is occurring to some extent.
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/rds-metrics.html

If your AWS account is subscribed to an AWS support plan higher than business support, you may be able to check for RDS with low usage by checking Trusted Advisor.
https://docs.aws.amazon.com/awssupport/latest/user/get-started-with-aws-trusted-advisor.html#view-check-categories
https://docs.aws.amazon.com/awssupport/latest/user/cost-optimization-checks.html#amazon-rds-idle-dbs-instances

The AWS RDS Console has a default metrics page that will show you the common metrics in a dashboard such as CPU utilization, IOPS, memory utilization, etc. Just click on the monitoring tab from the database detail page.