i was wondering could the mr16 dim the 12v lights
No, not directly.
The problem is that MR16 controls DC lights only. The 12V landscape lights you reference appear to be AC (in my experience, the vast majority of "kit" landscape lights are AC).
There are options, though (but *only* appropriate for a DIY "experimenter")...
First, you could replace the power supply with a DC 12V supply - like a computer power supply. The supply would have to be in a weather-tight enclosure (or in the garage or something), and appropriate "protections" would need to be in place (GFCI, etc).
Second, you could install a (high power) bridge rectifier & filter caps on the output of the 12V AC supply, basically making it an unregulated DC power supply.
Hope this helps...
--Dave
