Pronunciation is important for speaking American English. If your noun is not pronounced correctly then that makes more work for your listener. They might mis understand you. What you're doing is making your listener try to 'figure' out what you're trying to say or 'assume' what you're trying to say. It's much better to learn how to say letter sounds correctly so you can speak clearly and your listener will have a better and easier time understanding you. You could have amazing vocabulary but if the pronunciation of the words are completely wrong then it won't matter how many words you know.
If you have never studied letter sounds before like long vowels and short vowels... I can help! Book a 'new student' lesson of mine so I can help you sound clear and understandable to native English speakers.