The Gemara, commentary on the Mishnah compiled a century or two after the Mishnah, is indeed in Aramaic.
Yes, but the Targumim were in Aramaic. It should not surprise that Hebrew was still being used as a religious and scholarly language alongside Aramaic as the vernacular. We see the same thing with the former use of Latin and Old Slavonic by Christians.
So, you believe that there were Targums considered authoritative before the Mishnah was accepted?
The whole point of the Mishnah was to make it easier to learn and teach. Why not make it in Aramaic if it was the most common language? Why wasn’t it in Greek if the Septuagint was the accepted Bible?
We know the Gemara used the most common language of its day. Why the difference?