It's over is an expression used to indicate hopelessness. A common formulation is "It's over for _____-cels" with the blank being filled in with whatever obscure kind of incel is being referenced.
In the context of describing how Shrek was discriminated against once his real face was known, it's often expressed as "IT'S OGRE".
HeedandSucceed may be responsible for the usage of the term in incel forums. Although, the general sentiment can even be found in movies made prior to incel forums becoming well known, for example in extension du domaine de la lutte in this scene.