Do you mean the mtext window being transparent?
It will do this when the text in the window is similar in size as to when it will be placed in the drawing. This way you can see where to place it behind the window.
If you zoom out a bit and do your mtext, it should be fine.
You can turn off transparency by right clicking on one of your pallettes and selecting the transparency option and check the box to turn it off. This will affect your pallettes too, if you are using transparency for those.
