Okay, maybe "Clean Code" not as a new category, but as an optional feedback for the coders. You can review the code of others only if you submitted your own code and make it visible so that your code can be reviewed, too. So that could maybe reduce rating others code badly on purpose.
But then perhaps it could be that the coder feels attacked because of the bad feedback he got, even if the review is objectively right in relation to clean code.
One problem I have with short codes in clashes is that they mostly written in one line and almost no spaces between. I can't read such code but I would like to do but then you had to add space, tabulator or/and format the code so that is readable. But if you add these you will lose eventually. So one solution could be to ignore spaces, comments and tabulators to prevent one liner and provide better readability of the code. Here a link to a already existing topic in context to improving the short code clash: https://www.codingame.com/forum/t/clash-of-code-shortest-improvement/2765/4
Honestly, I get a little upset when I do Clash of Code and the winning condition is not just to write code that works but write short code. With Pascal you don't really have a chance because you have to declare variables and use keywords such as "program", "begin" or"end". I bet in some other programming languages you have some problems, too.
But I was also mad when I realised that I can't provide short enough code to acutally win in this category. Maybe it is time to learn a new programming language. I mean it is some kind of motivation for me but maybe a wrong one.