If you stay within the jetstream, it is a surprisingly smooth ride - a bit like being in a canoe in a middle of a fast flowing current in a river. Jetstreams tend to be fairly long and continuous.

Where it gets bumpy is if you are transitioning in or out of the jetstream zone - a bit like the abovementioned canoe hitting some rapids in a shallow part of the river.

No it's not. The airplane just has to do wind correction by either crabbing into or away from the wind depending on the wind direction.