You can't really boil it down to simply carbon, aluminum, titanium or other. Nor can you say this component over the other component. You have to know the weight of what you are replacing and compare it to the weight of what you are getting.
Generally on a road bike wheels will allow you to shed weight quick. But some carbon wheels are as much weight as good alloy wheels. Also, a complete change of the group set can shed some weight too if you go from mid tier to high tier group.
For your mountain bike I don't know how begin to consider wheels. As if you actually ride it in the conditions it's made for, then you want strong wheels. Alloy or carbon.... I wait to see what other say for wheel choice on a mountain bike for riding mountain bike conditions.
As for those Ultegra cranks, where did you get that number? On Shimano's website the current Ultegra FC-R8000 shows as 690g (1.52 lbs) for the 53/39 ring version.
Also, Ultegra is a road bike group. Might fit your mountain bike, but might not. You should be looking at mountain bike components. Though if this is an old bike, then probably a lot may crossover.
If you are trying to make a mountain bike a road bike. STOP. DON"T. It'll never be light enough. At most just put some slicks on it and ride it till you get in decent shape. Then take the money you saved by not spending on upgrades and get a new and lighter bike. Whether that is a road bike or mountain bike or other type.