If a people group tends to eat a lot of something strong and aromatic, they tend to smell of it. If Italians have a garlicky meal, they’ll smell of it for a while. The smell of curry is on East Indians who eat lots of curried foods on the regular. Someone who drinks coffee all day is going to have coffee breath to go with it. It’s a description of cultural cuisine, nothing more. (Shasta eats onions on the journey north, so he would have smelled of onion too.)
Lewis’ initial audience was British, and Narnia is described as generally European in flavour — literally. Differentiating a country foreign to Narnia (and therefore England) via cuisine is only natural, and he does that elsewhere too by naming exotic items (such as dates), stating that Calormenes use oil rather than butter (true of the nations that have lots of olives and not as much cattle, like Israel), and by describing the meal Shasta had in Tashbaan (containing things he doesn’t include in descriptions of Narnian feasts, like lentils). The smell of onion and garlic is simply another layer.