Not always. For example, in "This is Y" sentence Y is in Nominative, and even in "X is Y" sentence Y is in Nominative if it's just an adjective.
In fact, you can say that Y is in Instrumental if both X and Y are noun phrases (or X is a personal pronoun and Y is a noun phrase).
So, as "jutro" is an adverb, that doesn't work. In fact, it would be like defining "tomorrow" by being "a Saturday". Which is not true. Tomorrow happens to be Saturday, but it's not like every tomorrow is a Saturday.
More info here: https://www.duolingo.com/comment/16373167