Jumping the shark or jump the shark is an idiom, first employed to describe a moment in the evolution of a television show when it begins a decline in quality that is beyond recovery. It comes from a moment on the TV series ‘Happy Days’ in which the character Fonzie jumped over a shark on waterskis, and from then on (supposedly) the show bore no similarity to its original form.
Thus, in its initial usage, it referred to the point in a television program’s history where it has “outlived its freshness” where viewers feel “the writers have run out of ideas” and that “the series has lost what made it attractive.” These changes were often the result of efforts to revive interest in a show whose audience had begun to decline.
The usage of “jump the shark” has subsequently broadened beyond television, indicating the moment in its evolution when a brand, design, or creative effort moves beyond the essential qualities that initially defined its success, beyond relevance or recovery.
There is no exact expression in Spanish, but it could be similar to the idea of el inicio del declive tras llegar al punto álgido/ al clímax.
Here you can see the scene which gave origin to this expression:
Here you can see the expression used in the context of television programmes: http://www.tvguide.com/jumptheshark