Of course not. Being diabetic doesn't give you a license to perform poorly. This is between you, your union and your employer. If your termination violates your union contract, then it's up to your union to address the issue. If it's not a violation of your contract, then that's it - it's not a violation of any law.

This would actually be an ADA issue, not a FMLA one. But you're right, nonetheless - neither FMLA nor the ADA requires the employer to overlook poor performance.