What tool should be used to mud the inside corners of a diagonal ceiling?
I've had decent luck not taping inside corners at all. Instead I've use a sanded caulk. The key is to have a nice sharp edge to work with where the two pieces of sheetrock meet. 

But if you want to mud and tape, there are tools for that:



They come in various angles, but most can adjust the angle merely by angling the handle yourself.