I recently realized that you draw the “C” in the class circle during LaTeX export by yourself. Since LaTeX/TikZ vectorized, the diagrams might be rendered to any size. Sized to … maybe 2cm width, the “C” starts to look ugly. You might consider to just write a letter instead of drawing a C.
I tried to replace it with this:
\node at (20pt,24pt)[]{\textbf{\Large C}};
You can see the difference here: