Is there a reason April's mutation could be cured at least 3 times but not Splinter?
 

Aside from "Because that's what their toys look like" of course.

And before you say it failed in that one episode or that April was mutated in different ways, let's point out that Irma and Vernon were also turned into rat mutants with Krang's mutagen (even if there was an unexplained "instability" in that episode that was the cause behind the impermanence). That and there is idiotic crap like treating the destruction of the anti-mutation ray gun as if another one couldn't be built with literally no reason to think it couldn't be done or Splinter deciding that living a sewer is better than paying cab fare.

The Catwoman from Channel 6 was not a mutagen mutation.

I don't see any reason that Shredder and Krang's retromutagen ray wouldn't have been capable of restoring Splinter to human form. Perhaps Donatello was just not capable duplicating one. Or if he could have, hadn't Splinter accepted his new life in the form of a rat anyway?

Yup - there was a lot of 'story convenience' in Fred Wolf simply due to the nature that an ongoing anthology show meant daily 30-minute toy commercial. There was no thought to changing the status quo.

But sure, in any real-life situation the Turtles real goal would basically be to get the retromutagen ray gun at all costs.

Certainly the device that turned her into a fish mutant and then back into a human seems like it would have been usable on Splinter as well. For an in-story reason, maybe they were afraid Splinter wouldn't survive the transformation process from that device. It seems like it was a pretty painful process (April got hit with electrical beams and was screaming), so it might have been too risky to use on an older person like Splinter.

Because when you're really just a drawing it's easy to erase mistakes. :D

Okay okay, I know we're talking about FW. Um... I have no answer, cartoon logic is an oxymoron.


edit: Maybe Splinter really had accepted his life as is or felt it meant leaving the Turtles behind if he did go back to being human. Being able to have a normal income would have benefited them though. On the other hand a 9 to 5 gets in the way of important family matters.

"Hello? What's that, Donatello? Shredder and Krang might actually win control of the world this time and you need my help? Sorry, I'm at the office and this is a bad time, can it wait? I should be home around 6."

"Ah well, I guess it is my Karma to live my life as a rat. The important thing is that you are safe, and that we are together".

Krang should've outfitted the entire Technodrome in those guns. Problem solved.