I think it's a stereotype from the old web when it didn't require as much technical skill as other programming fields. Most of my colleagues started in other languages and learned Node/JS on the job.
Unless front-end development as a category has completely disappeared in the last few years there are certainly quite a few people who don't know how to do anything else.
Any language that is the most used is going to be used by mostly below-average programmers. I have yet to see a single domain where quality can be measured by popularity.
I didn't see anything on methodology, but I'm assuming they're just surveying their users—in which case, I think it would make some sense for javascript to be overrepresented.
The TIOBE index has always been a bit weird. If you read their note about their methodology, you'll see that it's measuring something kind of interesting, but not really what you'd call "the most used languages." For example, IIRC part of a language's TIOBE ranking is how many college courses use it.
Do you know of another more objective source, though?
Other folks are citing Github and Stack Overflow, but they are just reporting on proportions used on their own sites, and I would not be at all surprised to find that both those sites are more popular with web developers than say Java or C/C++/C# or Visual Basic devs (all those languages outrank javascript on Tiobe).