"Mathematics is an important discipline."
Translation:Les mathématiques sont une matière importante.
Firstly, I would like to know whether this a common use for "matière", and secondly, is it really common/acceptable to refer to "LES mathématiqueS" (plural) as "UNE matèrie" (singular)?
Yes on both accounts.
Matière for "academic discipline" is by far more common than discipline in the context of elementary school through high school and some higher education. "discipline" out of context is more readily taken as "regulation" or "self-control". It can also be used in the way you envision from this sentence, but would be mostly used in things like research, or somehow in sports (like track and field athletes can be competing in different "disciplines", or Michael Jordan may have been said to have "dominé sa discipline" ie be the best around in anything basketball)
Then the plural to singular works just like in the english sentence. French is not really more stringent on this than English is, as far as i can come up with examples to check off the top of my head.
There is a more precise explanation for "matiere" in Cambridge dictionary: subject.