DouWatch works without the use of a third-party server. There is no data being sent to me or any other third parties. DouWatch simply connects to TikTok in order to get the current trending videos.
There is none of your personal data being stored and read by the app. The only type of information that's accessed by the app itself is preferences such as font size, theme colors etc.
As no data is being stored externally and all data stays on your iPhone and your Watch, simply uninstalling the app will remove the reference between the app and its data.
For information regarding the connection between your iPhone and your Watch, refer to Apple's documentation (it's also really safe from what I can tell).