Yes, but not (yet) in a way you would expect
asciicasts are quite simple newline-delimited JSON files. You can edit them easily with any text editor which supports UTF-8. The recorded data is more or less a series of print statements, with ansi escape sequences (also known as control sequences or control codes). You can add, delete or modify text there. There’s no tool for visual editing of the screen contents though (due to incremental, state-machine based nature of terminal emulators).