I have had mono, so I understand how you feel. Every little thing makes to have to take a nap. It is so hard to get anything done.
First of all, take good care of yourself. I was in college when I got mono and I didn't rest like I should have. As a result, the mono and I will have to deal with each other for the rest of our lives. Three to four times a year the tiredness, sore throat and low grade fevers surface and knock me about for about 7-10 days. I know I have to rest during this time or I could get a full blown outbreak again. My doctor said this will be my life because I didn't allow my body to fully recover like I should have the first time around.
Second, mono is only contagious by saliva. You can't catch it just from being around someone with it. You have to either drink after them, eat after them or kiss them. Even then, some people will never get it. My husband has never had mono. For some reason, his body is able to fight the virus in a way mine didn't.
It is up to you if you want to decorate the cake or not. But decorating the cake (even if you had to bake the cake too) is not going to contaminate anyone, unless you start licking your fingers, spatula, bowl...and reuse without washing. That would be gross even if you didn't have mono, so I highly doubt you are just not going to do those things because of mono
I would suggest you ask the doctor. This way in the future if you have an order and you get hit with a small attack (like I do) you will know if you are okay or not do the order.