I rarely do the specific deity worship, preferring to claim my divine talents are a result of just being chosen by a particular set of deities that match my alignment of beliefs as the fluffy text suggest but I was curious if I was making a particularly heavily themed Dragon mystery Oracle, who shared at least one aspect of Tiamats alignment, and a strong desire for the destruction, and found that strength in the knowledge that even dragons of all kinds feared the mother of their kind as a source of strength would that be a good enough reason to claim to be her chosen?
You're quite free to claim whatever you want, especially as an Oracle, so sure.
I get the feeling Tiamat isn't that well known, in Golarion at least. She exists, but hasn't been involved in anything since forever ago - in this corner of the universe. PF has Dahak for that.
She would however be a great fit for the "almost forgotten yet terrifying", ancien evil niche. I kind of like her in that aspect, actually. The Queen is back, and all that.
Your GM is really the only person who could just plain tell you "no" about this kind of question. That said, it doesn't really make sense if you're going strictly by canon lore, and that might effect their answer. Tiamat is an ancient deity of dragons, but dragons don't worship her. They don't even talk about her. They (even the chromatics) hate and fear Tiamat that much. It's even implied that Tiamat doesn't know Golarion exists, which is why Apsu has chosen it as the site for his final battle against Dahak, to avoid her interfering again. So your biggest problems are going to be answering the questions of A: how does your character even know about Tiamat and B: does Tiamat know your character exists to grant them Oracle powers. If you can satisfactorily answer those two questions, I wouldn't see any reason for your GM to object to Tiamat as you Oracle's patron.