Мама is the subject of this sentence, so it's in the nominative case, not the accusative. Accusative is used for direct objects. There are no direct objects in the Russian version of this sentence, only in the English translation. The literal translation of the sentence would be something like, " (A) mother exists at (the) boy."
Nope, it's Genitive here in Russian. I also took a while to figure it out.
Case 1. Person x owns thing y. Only x is in the genitive (possessor).
Case 2. X owns none of y. Both x and y are in the genitive.
Case 3. X owns some (eg lots or a little) of y. Again, both x and y are in the genitive.
For cases 2 and 3, the trick is to imagine ownership of a share/ portion/ fraction of the whole. Then the Genitive makes sense.
Perhaps because "mama" in English is more a form of address than a descriptive noun like "Mom" = informal "Mother".
In other words, we use "mama" when talking to our mothers ("Mama, can you...."), but refer to them as "Mom" when talking about them. "My Mom is...."
That's not a hard and fast rule, but "My mama is..." is a little odd.