Worldwide there are many, many diabetics (and millions of them do monitor their glucose). And just like me, there are a lot of them that solely use Macs. Not everyone manages his or her diabetes well. Software can help with this as it allows you to see the trends better. And gives opportunities to add things like how much insulin you used and the meals you ate or sports you did. All these things help interprete the blood glucose levels and thus create a better balance
Then I agree with you .. there probably is a market.
I'd advise that you don't put your eggs all in one basket by tying the app to one particular device, although if it can hook up, thats a plus.
There are millions of diabetics, and a proportion of them will own macs.
Your app should allow connectivity, OR manual input of values (allowing use by people who rely on other devices or methods)
Take your visual cues from apps that help people monitor weight loss and exercise.
An app like that would benefit from being in the App Store.
However, sandboxing may make the connectivity difficult to achieve, so creating one without connectivity might be a good starting point.. if you get that to market, it could generate income to allow you to fund plugins etc for new more powerful versions.