This project is interesting, about the idea. But I don't really understand it. Can you explain me how it works?
Google: Mixing service bitcoin, and read some articles. You'll get the gist of it.
To get you started:
https://en.bitcoin.it/wiki/Mixing_service
Do you think this will be safe for hackers? Can they hack your wallet?
Unless Smartmixer somehow injects some sort of javascript into an extension or something, no.
If anything, it's far more likely they steal your bitcoins in the mixing process, since it is custodial.
(I'm not saying this is the case with Smartmixer (obviously not...), but this has happend in the past, and is still happening. Torwallet, bitblender, and countless of other small mixers.)