Is having m gravis a serious matter and if it safe to be a merchant mariner?

Yes. If you truly have myasthenia gravis, your safety will depend on whether you can maintain the physical strength required to do the job, and if you have an exacerbation it will not place you or others in danger. It will not slow your mind or reflexes. The word "gravis" literally means "grave", as "serious". Thankfully, it's usually manageable today. Glad you are eager to work -- good luck.
Treatment is key. Untreated, mg is serious and particularly in a potentially dangerous profession. Treated well, you will have a safe and happy career.