Complex sentence: The works of art displayed at these exhibitions were generally executed by avant-garde artists who had become recognized and esteemed in the 1920s, but the objective of the exhibitions was not to present the works as worthy of admiration but to deride and condemn them.
Split into simpler sentences: The works of art displayed at these exhibitions were generally executed by "avant-garde" artists who had become recognized and esteemed in the 1920s. However, the objective of these exhibitions was not to present the works as worthy of admiration but to deride and condemn them.