He does seem to be quite the leader, but there are very strong correlations between aircraft development and production (where Mulally came from) and car development and production. For Ford, Mulally is a person with deep background in design and physical manufacturing in addition to having great leadership qualities. Is it enough to just have leadership capabilities and weak roots in the core skills of the company?
Even if you say that Microsoft wants a strong future presence in hardware manufacture - building phones and tablets is a very different game than building planes and cars.
I still don't think one has to be the subject matter expert in everything a business does. Mulally should be an expert in building teams to make complex manufactured devices, and he should be hiring (or empowering) experts who understand services or tech functionality better than him. We can't hope that every leader can be a Steve Jobs or an Elon Musk :)
Even if you say that Microsoft wants a strong future presence in hardware manufacture - building phones and tablets is a very different game than building planes and cars.