Being a musical genius doesn't mean you're a visual genius. Especially when it comes to handwriting.
Today, the treble clef appears printed on composers' notepaper or preloaded on computers. It indicates notes' pitch, because it's placed on the second line of the musical staff, showing a G.
In the past, even the greatest musical geniuses often had to write out the treble clef themselves. The results were varied:
These clefs come from "autograph copies" of each composer's music — versions that they handwrote. Each is just an example, not necessarily how a composer always wrote. And the composers themselves were chosen just for flavor, rather than for any particular scientific or historical reason.
The chart above shows a mix of personal quirk and the evolution of a standardized mark over the course of a few centuries. As Smithsonian notes in its history of the sign, the treble clef evolved as a representation of the letter G (which is why it's also called the G clef, since it indicates that the line it appears on is G1). It probably began to appear in the 17th century, though other sources dispute the exact timing and origin, with some placing it as early as the 13th century).
Some of the treble clef's evolution can be seen in the timeline below, from Charles Villiers Stanford and Cecil Forsyth's 1918 A History of Music:
There's also something telling about the most beautiful treble clefs on the chart, which might say something about the artists' perfectionism — or lack thereof. One thing, however, is certain. Beethoven was very, very messy:
The best resource for manuscripts like these is probably the Petrucci Music Library, which contributed many of these manuscripts, as well as the British Library, the Library of Congress, the New York Public Library, and the Morgan Library.