Mixpanel’s documentation states that, by default, user data is stored on US servers via Google Cloud Platform.[EU residency]
You can see where your specific project’s data is stored (including residency and access URL) under:
Organization Settings → Projects → select your project → “Project Details” (field: Data Residency).[Managing projects]
The knowledge sources do not specify any use of AWS as a storage backend for Mixpanel project data itself; they only mention AWS as an option for exporting data via Data Pipelines.