It's today for us, "today" as the unit of time is still unfolding. So the baby is in the process of being good right now, this is a behaviour/action on her part that is still continuing as you speak.
You are not giving a characteristic that would apply everyday, be a constant or repeatable state; hence, Present Simple is not applicable.
Beside, phrases with "is good" are often idiomatic or are used as cliches in certain situations, so it's also partially about having a feeling for the language and not accidentally getting a totally different sense.
For example, before I clicked on this post, just from reading its title, I chuckled at "The baby is very good today" because it reminded me of phrases like "The salmon is very good today" (a remark at a restaurant where, the day before, you were served overcooked salmon). You don't eat babies, do you?
"I'm good" is often said, again, in situations where someone asks "Would you like more salmon?/another drink?", and you answer "I'm good" meaning that you are content with what you have and don't need anything else.
Without "today", I envision the phrase "The baby is good" to be appropriate in a situation where, e. g., someone asks you "Don't you think we should put another sweater on the baby before going outside, since it's so cold", and you answer "No, the baby is good".
"X is good" cannot be provided as a constantly valid characteristic (which Present Simple is used for), because this meaning would be handled by the phrase "She is a good baby (never whines, etc.)." Basically, the only context where it doesn't sound out of place is the phrase "God is good" (which is supposed to be a constant characteristic).
Everyone else "is a good something"—a good guy, a good cook, etc.
When something "is good", it's usually in the contexts I've described above.