Well, it's an odd sentence in English, but, without context, it should be accepted.
"The shape of it is correct" has the same meaning, and the structure of the Irish sentence is not the same as the English structure anyway
You could argue that "I have lots of money" and "I'm rich" mean the same thing, but that doesn't mean that you should translate tá lán airgid agam as "I'm rich".
Bear in mind that these exercises aren't designed to test your comprehension of Irish, they are designed to teach you Irish by exposing you to the way Irish sentences are structured. The tá X ar Y structure is very common in Irish, and it is worth teaching learners that it is often translated as "Y is X", and it's not just in sentences like tá brón air.
Interestingly enough, your suggested "The shape of it is correct", is an example of what you might call a "reverse genitive". To say "the shape of a ball" or "the shape of Seán", you would usually use the genitive in Irish - cruth liathróide or cruth Sheáin, and these could be translated back into English as "a ball's shape" or "Seán's shape". But you can't put a pronoun into the genitive, so both "the shape of it" and "it's shape" are translated using the possessive - a chruth - tá a chruth ceart. (I don't think you'd use the partitive genitive in this case).