Salad dressing is typically considered a heterogeneous mixture rather than a homogeneous mixture. A homogeneous mixture is one where the components are uniformly distributed throughout the mixture, and it has the same composition and properties throughout. Examples include saltwater or air.
On the other hand, a heterogeneous mixture has components that are not uniformly distributed, and you can often see the different parts. Salad dressing, especially vinaigrettes, often separates into layers of oil and vinegar, which can be easily distinguished. Even when shaken or mixed, the components tend to separate over time, making it a heterogeneous mixture.
However, some creamy salad dressings, like ranch or Caesar, may appear more uniform due to emulsifiers that help mix the oil and water components. Despite this, they are still considered heterogeneous mixtures because the components are not uniformly distributed at a microscopic level.