I know that mutation is going to be the hardest subject to understand/remember but would you be understood if you forgot to mutate occasionaly?
Not a great problem. Even fluent speakers will miss a mutation sometimes, and some mutations are becoming unfashionable in colloquial speech in any case, especially among younger people. (But some new mutations are starting up!)
Best to concentrate on basic language patterns and vocab, really. Mutations will become more natural to you over a longer period of time.