I assume that your question is strictly about font, and not about the overall layout and tidiness of your notes. If that's the case:
For framework, it certainly helps if you can write more clearly. Some interviewers prefer to look at your framework while hearing you talk. For them, it would be a lot easier to follow.
For general notes throughout the case, writing clearly is not as important, as long as you can recognize your own handwriting later in the case.
So, my suggestion is to try to write as clearly as you can, until you find it dramatically slowing down your pace or messing up your thought process.