According to Peyton Reed, if he's not dead, "he's at least severely hurt." Last seen imploding into the Quantum Realm in the most awkward, painful-looking way possible, it can be assumed that Yellowjacket is, for all intents and purposes, no longer among the living. At least until Marvel decide they want to use him again; if the director himself obviously hasn't made his mind up as to whether Darren Cross is alive or dead, it's safe to say he's currently the supervillain equivalent of Schrödinger's Cat. Corey Stoll is a great actor who maybe didn't receive the character he deserved to play; another appearance from his as Cross could go a long way to adding further dimensions to an ostensibly-decent Marvel villain (a breed of character that the MCU is sorely lacking, apart from the oft-praised Loki).