run only on posts listed under one category
I highlighted that “posts” because if so, then the conditional tag you should use is in_category() and not is_category() which is for category archive pages (e.g. example.com/category/foo) — so for example, is_category( 93 ) checks if the current archive page is for the category (with the ID of) 93, whereas in_category( 93 ) checks if the current post is in the category 93.
So try with:
<?php if ( is_single() && in_category( 93 ) ) :?>
<script id="mcjs">!function(c,h,i,m,p){m=c.createElement(h),p=c.getElementsByTagName(h)[0],m.async=1,m.src=i,p.parentNode.insertBefore(m,p)}(document,"script","https://chimpstatic.com/mcjs-connected/js/users/hidden.js");</script>
<?php endif; ?>