Yes and no. Punishing your kid for some dumb but innocent action has the potential to make them too cautious. Additionally, they might damage the toaster because they have no experience on how to properly open and clean it and that would make things much worse. That's especially true if your kid might be neuro-divergent - e.g. kids with ADHD tend to be clumsy but cannot help it, they might be 30% behind in motor coordination. So your 12yo might have the eye-hand coordination of a 7-8yo, and I would not let a kid that age clean a toaster on their own.

IMO, the best course of action would be to not ridicule or reprimand them but explain how this wasn't the best course of action and then clean the toaster together with the kid - that way they learn how to do it and don't feel like an idiot. And you don't have to fume over what your kid did wrong but spend some time together.

Obviously, if your kid actually IS dumb and reckless, things would be different.

I certainly agree that modulating the level of supervision of the cleaning to the kid's abilities is warranted, but I still think they should be involved to the end.

I don't see it as a punishment to make the kid stick around and help with the cleanup to the extent they can; rather, it's an attempt to practice conscientious habits (if you make a mess, you stick around to see it cleaned up even if you need help to do so), in addition to the specific learning experience that some things are really hard to clean, so we should be careful in our experiments with those things.

No, no I wouldn't disagree they should be involved, I maybe misunderstood and believed you'd make the kid do the whole cleaning as a lesson.

As cleaning a toaster most likely involves sticking things into it (like a brush, maybe even a knife to scrape off the burnt stuff) and you also don't want your kids to stick stuff (especially metal) into a toaster, I'd say it's safer to skip the "elbow grease" lesson in this special case.