OK Maki, let's use either "Isn't it" or "Isn't that" which will be correct in English, and the poll will tell us if the other users agree.
Pick one and edit, then I'll set a poll.
It's very difficult to say if this is correct. The original text doesn't make much sense. The question isn't in proper Dutch. Literally translated it would be "It's not". But that is not a question. And the second sentence is incomplete and could be translated as "But it's my ...".
This is of course an odd little half-sentence but it gives the impression there's something following 'mijn' as in Dutch we wouldn't say 'dit is mijn' (would be 'dit is van mij'). I think therefor the translation should be 'but it is my...'
In my opinion it's an abbreviated sentence (eg from a textbook how to learn dutch or something like that). there should be something behind the word 'niet' and since there isn't, any translation is a mere guess. so I'm guessing they want to say 'het is niet..(X)..? translating into: it isn't...(X)..?