I have been involved in many startups. Your question cannot be answered by us, because it's more like a risk management question: If something fails, can you fix it or make a workaround with reasonable amount of effort? And how many times do you expect it to fail?
In order to answer this question, you need to know what platforms you want to support, how you are going to use mono, and what skills you have. If you want to support all platforms, on which mono runs, you will run into many more problems, than if you just want to support Mono on Ubuntu, for instance. Mono is Open Source, but if you want to fix a bug inside mono, you need to have the skills and time to do exactly that.
Another thing is, how reliable do you want your product to be? If you look at a product like a Samsung's DVB-C harddisk recorder, it actually crashes sometimes. Even a Playstation 3 crashes once in a while. If you create an internet server, crashes can often be hidden well if the client retries after a timeout.
If I would need mono in a startup, and I would have at least 1 skilled programmer besides myself right from the start, I would have no problems with picking mono as the platform.