Can I adjust item frequency?
It is no longer included in the game. (Sadly)
You can merely adjust the items and whether or not they fall.
Here are a few sources I found, and while none have concrete evidence, no evidence pointing towards an unlock or hidden feature has been found.
http://www.gamefaqs.com/boards/632937-super-smash-bros-for-nintendo-3ds/70215054
http://www.gamefaqs.com/boards/632937-super-smash-bros-for-nintendo-3ds/70257251
http://smashboards.com/threads/change-item-frequency.371589/