Do they restock faster? Restock more? Get more customers? What about unhappy? Does that have any negative effects?
The happiness of your bitizens only helps you indicate if you've placed them in their dream job. An red unhhapy bitizen is either unemployed or employed in a job type which he has skill level of less than 5. A green happy bitizen is in his dream job. A yellow "meh" bitizen has a job and got at least level 5 skill in it's type, but it isn't his dream job.
Placing bitizens with appropriate skills and dream jobs helps you in the following ways:
Placing a bitizen in his dream job: Placing on the same floor one, two or three bitizens with their dream jobs will give you double the stock when stocking the first, second and third item respectively. This effect stacks, meaning that if all three workers have their dream job, all items give double the stock.
The effect of job skill: Bitizen job skills gives a certain discount on stocking costs, the cost is equal to the total skill level of all workers in the relevant type, for exmample, if you place in a Retail floor three bitizens with 5, 7 and 9 retail skills, you'll get a 21% percent discount when stocking the floor.