I don't recall if there is any flavor text for or against this, but is a Technomancer's Matrix Persona's appearance fixed, or can it be changed?
The Persona's icon can be changed, yes. They have a persona that follows the same rules as "normal" personas:
A persona is a firmware program built into the device’s hardware that you use to interact with the Matrix, in AR and in VR. It allows you to perceive the digital world and projects your icon (or icons) into nodes. Without a persona, you would be unable to access the Matrix. Even technomancers have a (living) persona.
All personas have icons:
Your persona is represented by an icon in the Matrix, as are your running programs, files, agents, IC, links to nodes, etc.
And icons can be modified through the Alter/Swap Icon, which is not specific to hackers, and available to Technomancers as well:
Alter/Swap Icon (System)
You change a detail of an icon representing your persona or one of the programs, files, or nodes you own, or switch the icon to a pre-rendered icon you have stored.