Is it always better for children to be taught in their "mother tongue"?

Under normal circumstances, yes. But it seems to me that the medium of instruction is not always the sole problem. There are other factors like school infrastructure, teaching materials and teacher training.

English-medium (or French, for that matter) is often blamed for poor achievements in many countries. But there are countries where the medium of instruction from Elementary 1 is not necessarily the L1 of the students, and yet have very high academic achievements. Singapore and Taiwan are good examples. Enough support existed for lower-achieving students in those countries, compared to their poorer counterparts.

Of course, the ideal is that everybody gets to study in their own language. But in societies where there are many languages, it is not easy to prepare teaching materials in all of them. As those countries often happen to be poorer ones, a lot of money that goes into developing materials in many smaller languages may actually reduce the money that can go to improving school facilities and teacher salaries.

In the meantime, students who come from richer families will always do better, regardless of what language they are taught in. They have the means to avail themselves of better support. So even if a country changes the medium into a local language, they might be disappointed to find that the economically privileged kids still do better all the same.