Well completely unrelated is maybe an overstatement.

English and Chinese is the common example of unrelated "mama"s, but there is a non zero chance that they are distantly related - beyond the time depths accessable by the comparative method of linguistics.

It could be that "mama" or a very similar word dates back to like Ancient North Eurasians or some older population ancestral to both Chinese and English speakers.

If any one word could survive that long, some form of "mom" is a good candidate, but yeah the most popular theory is that something about the "m" sound makes it more likely to be in the word mama.