Good manners have no relation at all to income and/or social status. Good manners are (should be) taught by your parents from day one. You do not need money to have manners.
2. Can the idea of good manners inhibit people's natural sense of fun and inhibition?
No; not at all. Some people might use it as an excuse, but that what it is 'an excuse'.
3. Do people have more manners now or in earlier times?
As much as it pains me to say this, people in earlier times had far better manners. Sadly it often seems and feels as if too many parents don't teach good manners.
4. Do you say "hi" to people even if they are strangers? Why? Why not?
I rarely go out, but when I do I usually acknowledge people, even if it's with a nod and or smile rather than an actual 'hello'.
5. Do you listen to other's people conversations on the street.
Again, I rarely go out so this doesn't come up very often. I don't set out to listen to a conversation, but there are times when one cannot avoid hearing what people are discussing.