Why do you think Orwell has Snowball make mistakes in his writing?
I assume that you are referring to how Orwell says that Snowball cannot quite write the letters properly when he writes the commandments up on the wall (towards the end of Chapter 2). In my opinion, Orwell does this to show that Snowball and the other pigs are still inexperienced and are not as "polished" as the human beings.
I think that Orwell is trying to point out that the animals are trying to set their system up from scratch. They are going to face obstacles because they do not really know how to govern. In fact, even the best of them cannot write 100% correctly. If this is the case, you would think that they are sure to have some problems as they set up their system.
Revolutions tend to be run by people who have not governed. This means that they do not really know how to run a government. That may be one reason why revolutions often go bad. I think this is what Orwell is trying to get across by having Snowball make those two mistakes in writing.