I may like the person I'm standing next to at Mass very much - but there is a time for small talk and Mass is not that time.
The celebrant may have a winning personality, but his personality is completely irrelevant to the Mass.
I concur with all who are saying "Hey, there's a time and a place."
Seriously, nobody's saying "Good Morning" is a mortal sin, it's just inappropriate to a solemn occasion.
How would you judge the propriety of starting a U.S. Marine Corps Flag Ceremony with "Hi there, everybody"? or ending a funeral with "BUH-BY!!"?
Better that than “hi, y’all”, right?