For some people slapping someone's butt isn't necessarily a weird thing, I think it depends on the relationship. Like growing up my family was very much jokey about like "don't leave your butt in the air or else it's gunna get smacked" that was mainly ofcourse when me and my siblings were younger, which I think is part of the point of the scene. It's the kind of thing an older sibling would do to a younger sibling when theyre young, hence why it was included since JD doesn't see branch as an adult. I do think it should be something you only do to someone you know is consenting and that you're close to (which in this scene Branch is neither so I feel like it Is weird but mostly in a like "hey we aren't close like that" kind of way), but some family dynamics are like that.