Including phrases separated by commas or semicolons is a technique that mimics the frequent leaps of thought in Clarissa's mind. The list of items exposes her excessive mental activity. The disparate phrases also suggest that Clarissa's attention is divided among multiple competing subjects.
In the quoted lines, the technique seems to suggest activity and movement both in the scene outside and within Clarissa's mind; it blends the bustle of the external world with her internal state. Though the phrases are disparate, their combined effect is to show continual activity in Clarissa's mind and to suggest that the moment is alive.
If you view the phrases as direct representations of Clarissa's thoughts, you might argue that most people don't think in such strings of phrases, and there is no reason to assume that she suffers from a mental condition. Therefore, the technique makes the description seem contrived.
However, it's also possible to see the disparate phrases as a list of all the sensory input that registers in Clarissa's mind (sometimes mixed with descriptions of her thoughts). If the lines are considered a series of images, then the technique does not seem contrived; the phrases capture a passing scene or a moment in time in a kind of sketch that comprises both sense perceptions and descriptions of thoughts (but not actual thoughts).
