Sending Events to Mixpanel requires users to be engaging with your app while it is open. So if your app is closed and users choose to delete or uninstall it, there is no direct way to track this activity with Mixpanel (though total numbers of uninstalls should be made available to you via the App Store and Google Play).
Proxies for app uninstalls: Churn
While it’s not possible to track specific app uninstalls with Mixpanel, you can track churn (i.e., users who are no longer active or engaged with your app) in other ways. Inactivity is a good proxy for, and likely should be treated the same as, uninstalls, as both indicate the user is simply not using your app:
Use a date People Property, like “Last Logged In,” to track users who are no longer active in your app and monitor this Property over time.
If you’re not already collecting last login data, Mixpanel automatically creates a property called “Last Seen,” which updates every time a People Property gets updated. You can consider users whose “Last Seen” date was more than n days/weeks/months ago to have uninstalled.
Use the Retention report to look at which Events in your app result in users coming back over time and which do not to understand which features may be resulting in uninstalls.