MMA is a compound we all have. It is metabolized AWAY by enzymes using B12. There is a birth defect that causes it to build up in babies (broken enzyme), which is fatal. There was a landmark child abuse case where a mother was charged with killing her child who had this defect. She was convicted and later discovered to have this problem and she was released from prison.

So when MMA is ELEVATED that is a serious thing, and means the B12 is not working or not present in high enough amounts to work. It is the reverse of what one would expect from tests.
Low MMA does not mean a "deficiency" where B12 is concerned.