Lucy's monomania is developing in this strip, but I link it mostly because you get a really good look at her clothes in this one.
Lucy wore two primary outfits that I know of in Peanuts' run. This shows her earlier outfit and the one she wore during most of the classic period, a slightly formal number with a bow in the back, and colored blue on Sundays. Here the skirt is fairly normal, later on it would become highly stylized and stick out almost at right angles to her body. In later years, probably as a nod to changing fashions, Schulz would adopt a kind of jumpsuit for her attire, which made her look vaguely more athletic.
She still has the round eyes here. I never get tired of pointing them out. They won't be here much longer. When Linus shows up, which is in a month or two I think, he has the same eye style as Lucy but from the start he has her parenthesis, or "Binkley," eyes.