The first joke is just a way of expressing our dislike for politicians. Try telling it to somebody. they say 'no', expecting an explanationg and get the unexpected reply 'good'.
The cartoon doesn't sound that funny, but it depends very much on the picture. Clearly one person is meant to be completely different from the rest.
Humour is infectious because one person's laugh can start another person laughing. 'Wit' is the ability to make rapid jokes, especially playing with words. So if someone's wit is infectious, it spreads to other people.
'Home folk of rural America' sounds as if it's meant to describe people of conservative tastes.