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If 75% have diabetes (or don't have diabetes -- either way), then 75% accuracy is also the accuracy of a naive coin flip.

That is to say, don't compare to the accuracy of an even coin flip; compare to the accuracy of a coin flip that flips according to the proportion of the people with the attribute you care about.

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> If 75% have diabetes (or don't have diabetes -- either way), then 75% accuracy is also the accuracy of a naive coin flip.

I don't think that's right. Always answering yes is going to have 75% accuracy if the prevalence is 75%, but always answering no will have 25% accuracy, and the coin flip will pick both of those equally (and uncorrelated with whether the individual has diabetes or not).

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