Looks that way.
The 49ers were 8th in total defense this season.
They kept KC down, for the most part, in the Super Bowl.
Sure, they gave up the last drive when the season was on the line, but it was Patrick Mahomes, and he was in 4 down m0de.
Fired?
Maybe there were some "internal" things we don't know about, but on the surface, it seems like a scapegoat firing.