A Bitizen's 'Dream Job' is their preferred job in the Tiny Tower. If a Bitizen is placed in their Dream Job, they get the green mood emoticon. Also, you can get up to five Tower Bux. Like many other aspects of Tiny Tower, the Bitizen's dream job is randomized. Dream Jobs can be changed using the Celebrity VIP from 3.04 onwards. Having bitizens in their dream jobs doubles the amount of stock you receive on each item per how many dream jobs you have in a specific job. So for example if you place 2 dream jobs for a store, then the first two items would get double stock, but the third wouldn't. It does not affect the discount you receive on items; the skill level of the Bitizen placed in the job gives you a discount, regardless of it being their dream job or not.
That means it's more important to have the Bitizen's in their dream job than the max skill, is more worthly 0 skill and dream job than 9 skill without dream job.