A few suggestions, although none is a direct replacement:

• Atteindre son apogée: A place from where you can only go downhill. However, unlike "jump the shark", the term is positive.

• Tournant: A significant change in direction. Might be positive or negative.

• Partir en vrille: Lose control.

“This is Trump’s jump the shark moment,…” might then be translated to:

C’est un tournant pour D. Trump,…

À partir de là, D. Trump a commencé à partir en vrille,…

After reading the "jumping the sharks" wikipedia page, and @jiliagre’s answer, here are other examples I would like to add (I am not completely sure about my translations though) :

• A un moment donné, il ne savaient plus quoi inventer

At one moment, they ran out of ideas [hence : they started doing stupid things].

• A partir de là, c’est devenu n’importe quoi.

From that point, it became nonsense.

• A partir de ce jour là, ça a viré au (grand) n’importe quoi.

From this day, it went (full) nonsense.