"Is it possible to set the position of a uialert within app designer?"
As of r2019b, no.
Alternatives include using a msgbox(), errordlg(), or warndlg() but this comes with a different set of issues. Unlike uialert(), you'll need to compute the possition of the dialog box so it's on top of your app and you'll also need to set the modal properties in order for it to behave like the uialert(). Note that the Matlab documentation recommends using the uialert() over these other methods for AppDesigner apps.
Another alternative is to reserve a text window within your app designed to provide feedback to the user. In several of my GUIs I have a separate function that receives as input 1) the message, 2) the message type (warning, error, info, etc...). The text area is updated when this function is called and the app behaves differently for each message type. For example, error-types make the font color of the message red.
