Not sure about this. The מצוין really emphasises the excellence of the meal better than just a mere "great" (which of course is still above average).
I guess there are probably many translations for "מצוין". My Hebrew Teacher from Israel told me that "מצוין" could also mean "special", but "special" is not accepted on duolingo.
Why does metsoy"a"n מצוין change to metsoy"e"net .
Is there a rule for the change in vowels when a noun or verb changes to another gender or becomes plural ?
This is very common:
perfect - mushlam/mushlemet
online - mekuvan/mekuvenet
By the way, it's metsuyenet, metsuyan, not metsoyan/enet.
It's always spelled with a ה but how you pronounce it (either with a -h- sound or without) is generally dependent on regional accent or how old you are. Most modern Israelis won't pronounce the -h- but from my experience the -h- sound is prevalent among the older generation and among non-native Hebrew speakers.