The numbness means that the mental nerve is not working. This will usually go away in time - which may be 6- 18 months You should see increased tingling and other sensations as the nerve recovers.
Tingling and numbness of the lower lip and chin are possible after placement of a chin implant and in most cases, this issue resolves completely over time. Sometimes the numbness will only last a few days but it is possible that it can last a few weeks or months. In general, it should be improving over time though. If you do not feel that the numbness is improving at all, you should contact your surgeon to be evaluated.