Bicycle rims are, in general, equally strong on rims intended for "mountain" or "road" use. The differences in rim extrusion cause most of the difference between otherwise equal rims.

Wheel "toughness" has a lot more variables- number of spokes, spoke tension, amount of offset, skill of the wheelbuilder. Tires are another variable- the greater volume of "mountain bike" tires allows for more cushion against impact and thus a lighter rim can be used.

There's many, many more aspects of this subject. Do you think you can ask a more specific question?