Good idea! This is one of those things where the underlying technology around voice recognition could be used for it, but not sure that the Echo Show is a great fit.
When building skills for the Alexa, you need to define a dialog between the user and Alexa. It means that you need to guess around what possible phrases could be used, and then it could write those out to the screen. It doesn’t have an option that allows anything that is uttered to be written to the screen.
Now the underlying speech recognition could be leveraged with a tablet, and Amazon has recently released those services to build with. It just won’t be a great fit for writing for the Echo Show.