Ahsoka showing up would be fine even after all the reasons Miguex stated, but her destroying the droids wouldn't have been. She showed earlier in the season she's a serviceable jedi knight who can go toe to toe and beat someone like Mando, but needed his help to take on a tough force. Luke Skywalker showed he is a jedi god who can roflstomp groups of rando droids, two of which would have massacred Mando. If she came in and demonstrated anywhere near the level of destruction that Luke did, it'd be an unrealistic power boost and be confusing for the viewer.
They could've had fuckin' Mark Hamill up there with a clean shave, a haircut, some makeup and with zero CGI and it would've been better than Ahsoka.