I'm at a hotel on their wifi and getting pounded with mDNSResponder pings and it's irritating as F*&%k to have that notification keep popping up every few seconds -- if I disable it in MacOS some of my browsing and email stops working
Incoming UDP connections are denied, if no rule exists. Instead of suppressing the notification, just make a rule. If you make a deny rule, it's the same as just suppressing the notification. If you make an allow rule, automatic discovery of computers and other resources in the local network will work (for the price of weakened security).
That's another possibility. If you just want to silence notifications for incoming connections, make a rule for them in Little Snitch. Without a rule, incoming connections are denied. So, if you make a deny-rule, the behavior is unchanged, but no notification will be shown.