This is not a bug, nor a feature request. Let's call it an inconsistency.
If I put up a tagcloud in an aggregate blog, it will still show only the tags from posts originally in that blog. This in contrary to the Posts list and the Comments list
I think the tagcloud has more use when it shows all tags on the blog.
Being dealt with in this post: http://forums.b2evolution.net/viewtopic.php?t=15779