I don't see a reason it would not be appriopriate to type-write a thank you note and mail it as a letter.
As a matter of fact it is a well know and most commonly used form of post-interview thank you note in a business world.
I type-write all my thank you notes and sign them at the bottom.
I make sure that they are not "generic" though. I always include at least a paragraph that makes each letter personal and unique (eg. describes something my interviewer mentioned or did during the interview or something we discussed).
Oh and one more thing: if I do send more than one letter to the same institution I make sure that my "templates" are all different 😉