"I'mma be" is mumbled English for "I'm going to be". It's not an idiom; it's a lazy form of speaking. No, it's not common. People would look at you very strangely if you (a learner of English) said "I'mma". You'd sound like a wannabe rapper.

No, it's not common at all. As a matter of fact, it's not even a correct phrase. It's a phrase the songwriter simply created to make the lyric work well.

It's short and really bad English for "I will be" haha. I would be careful with trying to repeat English you learn from song lyrics. But it definitely is good to improve your hearing skills.