@dominicr
The MDRs on NCL are "free", covered by your fare, so, unlike Specialty, where you either pay or have a "voucher of one sort or another, you have an account to sign even if you are not getting charged for anything; there is nothing to sign
I would say that it is VERY uncommon to tip in an MDR. Even in a Specialty many don't as they have paid a 20% Service Charge.