I faced the same bug on one of my site under construction.
Tried ST plugin for tag cloud, but it not helped much.
Googled a bit, and found that people who’s using simpleX theme usually don’t use tag cloud at all, because cloud is buggy in 50%.
I believe that the bug is in CSS, in a and a.hover description what applied for whole sidebar, include tag cloud. And as follow – ruins whole cloud.
And also there is no class description for tag cloud in CSS.