Diabetes, type 1, here. It's an interesting issue for valve replacement patients. Consider first the mechanical valve scenario. On one hand, diabetics should be perfect candidates for ACT and home monitoring, being well versed in diet and blood testing. But, on the other hand, adding one more monitoring scenario to the mix might just make both less successful, depending on the commitment of the patient.
Diabetic complications overlap valve replacement issues also, things like stroke, kidney failure, and retinopathy. On one hand, perhaps a mechanical valve and ACT would not mix well with stroke and retinopathy risks. But, on the other hand, kidney failure wouldn't work too well with a tissue valve and additional surgeries.
My two cent opinion is that control of the diabetes is the most critical component, and doing so and minimizing the risks of diabetes will help take care of most problems for any valve selection. I would recommend meeting with key specialists (such as endocrinologist and ophthalmologist) prior to finalizing valve choice, though, just to make sure you have considered everything fully. For example, my surgeon brought up a general concern of ACT and retinopathy. However, my ophthalmologist told me that it was a non-issue since my diabetes was under good control, and I had no traces of developing retinopathy.
