My personal sign is when you start seeing awkward collaborations start cropping up. One time when I was thrifting, I picked up a graphic novel that had the Justice League, team with the Power Rangers of all things. I glimpsed into what the plot was about out of morbid curiosity and it was just a plain generic time and dimension thing.
Nothing ever connected between the teams at all. DC Comics, while fledgling at times with how they go about their series and movies, still have far more relevance than Power Rangers do. I think the Power Rangers are just grasping at straws to keep being relevant when people have largely moved on from them.
Superhero comics are one of those things where I don’t think it’s possible to jump the shark.
The Justice League itself was kind of an awkward collaboration starting back in the 60s where they brought together a bunch of disparate different comic characters into a shared universe.
That being said, I think a series has jumped the shark when it becomes entirely unrecognizable from its original iteration to the point of absurdity. You would never expect to see a scene where The Fonz jumps over a shark while water skiing if you only saw the first episode of Happy Days
I think with long running superhero comics it is more like, if a specific run has jumped the shark and gotten too stupid.
What is simultaneously good and bad about long running comics is that the continuity is so convoluted that the writers can reset it after an especially bad run, or they can go do stand alone stories; and readers can just ignore entire chunks of continuity they don’t like.
That’s how I feel about Star Wars movies. And Highlander (there can be only one)
Which is fair, but you could also say the same thing for the Avengers. They both started out weirdly like making you think “what the hell do these guys have in common with eachother?” at first. Overtime, as comics developed, they became more established. Then it became more accepted and the rest is history.
But sometimes just pairing them up with say, my example, it makes it even weirder to where nothing meshes.
I would argue that the Power Rangers are not that odd of a pairing for the Justice League.
Quality of the work aside, both are superhero teams with complicated lore. They could use that highlight differences/similarities in ideology or methodology between the characters to create drama, before setting aside their differences to come together against a shared evil. The bones of a good superhero story are there.