Not sure you can state that categorically, like that.
I think that it is entirely dependant on other variables.
If the dev still completes a full and satisfying update, as well as the crossover, then I'd agree.
But, if the dev takes time away from developing story &/or character progression in this game, instead using it to create a scene for an imported character from another game who adds nothing to this story or to any of the characters...
Think that it is, effectively, dilution.
Some crossovers dilute nothing, sure.
Some absolutely do.