Based on the comment in the issue on GitHub - https://github.com/SharePoint/sp-dev-docs/issues/10039, the SPFx Form Customizer will not work for the case SharePoint Tab with List view used in Teams
However, there is a workaround how to add customized Modern Forms in the Microsoft Teams desktop app:
- Create Modern page in SharePoint
- Add list web part to the page
- In the Microsoft Teams desktop app add this page as a tab