In its simplest form, you want to create and "alert" when a value is > a "limit" right?
So add a switch node and set the comparison type to
> and the value to 30 (or whatever equates to 30%) then feed that to a switch node that sets payload to a suitable string e.g. "humidity is greater than 30%" and feed that to a notification node.
In basic terms...
Sensor (MQTT input) switch node (test the value) change node (set payload to your notification message) notification node (so that you get an alert)
Notes: this is pretty useless to be fair. You'd be better off sending an email or telegram so you don't have to sit with your eyes pinned to a dashboard.
Better still, you would feed this "alarm" to a system that controlls the humidity (perhaps stop water flow or an air conditioner to reduce humidity) and have the system look after itself