A damaged sticker is invalid, the resulting penalty when caught is equally to driving without a vignette.
Check penalties and fines here.
Related articles:
What happens if I drive without a Swiss vignette?
Where can I buy the physical Swiss motorway vignette (Sticker)?
How much does the Swiss Motorway Vignette cost?
Can I get a refund for a destroyed Swiss sticker?
Can I remove and reuse a physical Swiss vignette (Sticker)?