Was Crocodile stronger at Marineford? Or was he holding back in Alabasta?

 During the Alabasta arc, Crocodile displayed a level of power that was initially considered overwhelming by the Straw Hat Pirates. He possessed the Logia-type Devil Fruit called the Suna Suna no Mi (Sand-Sand Fruit), which granted him the ability to control and transform into sand. He had a reputation as a Shichibukai and controlled the desert kingdom of Alabasta from the shadows. His strength was showcased through his battles with Luffy and others. At Marineford, Crocodile was present as part of the war that took place at Marine Headquarters. While he did participate in the battle, he didn't display the same level of dominance as some other powerful characters present. This has led fans to speculate that he might not have been as strong as initially portrayed in Alabasta. It's important to note that power scaling and character abilities can be subject to interpretation and development by the author. Oda often keeps details deliberately open-ended to keep the story intriguing.

Could advanced technology or genetic engineering ever be used to create something like a 'monster' in fiction, such as Frankenstein?

 It is unlikely that advanced technology or genetic engineering could be used to create a monster similar to the one depicted in the novel Frankenstein by Mary Shelley. The concept of a "monster" in the book refers to a being created through scientific means that is subsequently rejected by society due to its grotesque appearance. While it is theoretically possible to use technology or genetic engineering to create unusual or abnormal physical features in an organism, it is highly unlikely that such a being would be created intentionally or be able to survive outside of a laboratory setting.

Additionally, the moral and ethical implications of creating a being specifically for the purpose of being rejected or ostracized by society would be highly questionable. The field of science and technology has made many advances in recent years, but it is important to consider the ethical implications of these advances and to ensure that they are used responsibly and for the benefit of humanity.