Scout means that Uncle Jack smells like alcohol and sweets, literally, but she means he is not quite domesticated.
The purpose of this description is to give us more insight into Uncle Jack, Atticus's younger brother. Scout likes Uncle Jack, because unlike her father the hard-working lawyer, Uncle Jack the doctor is a little rougher and a little more uncivilized. He never married or had children, and just lives life on his own terms.
The description of Uncle Jack is a very parental one, but he is more uncle than father.
Uncle Jack went to the livingroom and sat down. He slapped his thighs for me to come it on his lap. I liked to smell him: he was like a bottle of alcohol and something pleasantly sweet. (Ch. 9)
The simile is appropriate, because he not only smells like alcohol and sweets, he seems to literally be alcohol and sweets. He is a little rough around the edges, and a little bit of a wild card. Although he is a lawyer, he never married.
Uncle Jack also spanks Scout for fighting with Francis without really investigating the situation. This shows the more irresponsible side of him. He is in over his head, and doesn’t really know it until it is too late. He does try to rectify the situation though, and listens to Scout’s lesson on children.
Uncle Jack put his hands on his hips and looked down at me. "And why do I not understand children, Miss Jean Louise? Such conduct as yours required little understanding. It was obstreperous, disorderly and abusive-" (Ch. 9)
Once he finds out what really happened, he is more upset at Francis than Scout, because Francis picked the fight with Scout for Atticus defending Tom Robinson. However, when asked what a whore-lady is, he tells her about the “Prime
Minister who sat in the House of Commons and blew feathers” (Ch. 9), which later earns him another lecture, this time from Atticus, about giving children an evasion instead of an answer when asked a question!
This short description tells us a lot about Uncle Jack, actually. We already know he is the younger brother that Atticus sent through school to be a doctor, and that he never married, and that he lives with his cat. Now we know more about his personality too. It makes him seem personable and a little extra human. Jack has some more faults to make up for the lack in Atticus.
Uncle Jack, the bachelor who lives with his cat and asks Miss Maudie to marry him every Christmas, is a scholar and somewhat of an enigma. While he may like his drink a bit too much and eat a little too much candy, he is on balance a nice guy, and a soothing influence in Scout’s life.