1) It's a cultural thing. MAN-SOF for instance, many of the Bulgarian passengers will applaud.
2) Not as much these days but it happens a lot on leisure flights. It's not necassarilly a class thing nor appreciation deirected towards the pilots. It's because people are going on holiday....they're happy and cheerful. It very rarely happens on the inbound sector. Go figure.
There is an awful lot of snobbery when it comes to this. As crew, it makes me smile and usually means that pax are in a good mood which by default means they have had a good flight. It harms no one.