Actually, "Related" is wrong in case you view the whole list.
I'll start from the actual use of that "widget": when you're viewing a specific tag page e.g. c# the Related Tags lists tags having most questions tagged with both the tag you're viewing and other tag. So for example, if the list for C# tag contains this:
It means there are 12,782 questions tagged both c# and wcf so Related Tags are "tags that frequently appear together".
Now when you don't view any tag (i.e. main questions list) it just show the top tags, sorted by amount of questions, same order as in the tags page, popular tab.
So changing to "Top Tags" or "Popular Tags" when not viewing a specific tag is a reasonable request.