How do you say exceptional mother in Latin matris eximius or mater eximius?

Answer 1

I'm a Spanish student so I'm going to put the adj. before the noun it describes. Not that that's at all your problem.

Don't change "mater" to a genitive. Unless context for some reason dictates that (in which case your adj. would need to change case too).

Being and adj. "eximius" also needs to match gender with "mater". It should read "mater eximia" or "eximia mater" (if everything is in the nominative singular).