Following the suggestion from @Johani here:
To which Johani replied:
Unfortunately, wiki is a post-level, not a topic-level property. So, It’s not added to the topic list item model. You can use a tag or create a feature request for Discourse to add that class.
I suppose the vast majority of wikis are first posts, but I understand the post level issue.
However, having the possibility of emphasizing wikis on the topic list would be a neat addition and may incite people to visit them and maybe participate in their content.