You'll hear announcers call it a muffed return even on a kickoff, it's just way more rare because muffing a kickoff is more rare. The other thing is that muffing a punt changes the punts status from a kicked ball to a live ball for the kicking team, whereas a kickoff is a live ball for the kicking team once the ball travels 10 yards or the receiving team touches it, hence more emphasis.
You could call an onside kick that travels less than 10 yards and touched by the receiving team a muffed return, but muffing implies clumsiness rather than just chaos occurring.
For clarity: a punt is considered the receiving team's ball until it's touched by the receiving team and once it's touched it becomes a live ball, because a muffed punt was never possessed by the receiving team it can't be considered a fumble, because a fumble requires a player to actually possess the ball.