It's definitely not a term of affection. The Joads were not objects of mockery; the infrequent flyers FT often terms "Kettles" definitely are. The Ma and Pa Kettle movies made fun of their principals.

It's a belittling strategy in the name of tribal gatekeeping, although I will never understand why people feel the need to belittle those who don't know as many acronyms, fare classes, etc. as they do. It's not very nice.