I would give this episode a 6.5/10. It was very "meh".
The highlight was most definitely Dumb Donnie. Everything he did was hilarious and stuff I'd never thought I'd see Donnie do. What was the Pinky and the Brain reference that some people have said is in the episode and some have said it isn't?
"I like French Fries!!" should have been on that voting thing at the top. I was laughing so hard! Or maybe "Oooo...Squirrellll!"
I just wished Dumb Donnie was the sole conflict of the episode. It was funny, entertaining, and new, so naturally I wanted to see more of it. Because of the Creep/Snakeweed marriage, this Dumb Donnie scenario felt like a wasted idea that could have had so much more laughs. While it was nice to touch up on some old villains (not that I care for them, but the show needs to refresh its memory on the bad guys sometimes), think about what an episode with Dumb Donnie as the sole conflict would be like:
- The problem could have arose from something besides the default "Mikey Makes a Mistake" trope
- There would be full detail of Donnie's slow descent as the others rush to cure him before it's too late
- Tension builds as Donnie goes from merely saying dumb things and being absent-minded to actually becoming a danger to himself, while completely forgetting who his family is
- A plot twist as Donnie becomes so brain-dead that he starts to attack his family, literally "fighting without thinking"
- More time for Mikey to create an actual cure, intelligently reading Donnie's notes and taking some guesses, as the others hold back the relentless Donatello
- A final fight scene with Donnie, with April managing to calm him down with her powers while Mikey administers the cure
Overall, I find that idea much more interesting, with some tweaking. Fillers can and have been great.