I don't believe this is an issue so much with MobileJoomla as it is with the blackberry native browser itself. If I try to view my site using the blackberry browser it will display properly, but it takes a long time to load (or actually process I'm guessing). When viewed via Bolt browser on the same device the page loads much faster (and looks better there too). This happens both using wifi or using the data connection over the air.
I tested the same page on an iPhone and iPod touch here on our same wifi network and they loaded much faster. I think it may have something to do with the poor way the OS5 blackberry browser processes css and scripts. I will be testing BB OS6 soon so I will post up more info here.