Is it okay to use 30w adapter charger for iphone 12 pro max

As long as it's 4.5 volts (I'd be surprised if it wasn't) and the connector is the same* it's OK. You won't overload the phone, any more than if you plugged a 5 amp heater into a 13 amp socket: devices draw the current they require.

• The iphone probably has a different socket from an ASUS, but the charger end of the lead is normally. Thie charger probabll shouldn't be a fast one..

Yes, as long as the voltage is the same (most are): the phone will take just the power it needs.