Good idea, but AFAIK some wallet already use /dev/random and few OS-level random which generally accepted for cryptography usage.
Additionally, /dev/random have various entropy source which is more "random" than mouse.
The point is to add an extra set of entropy on top of using /dev/random and whatever other RNG, it's just to spice things up after you've setup your whole thing, obviously you can't depend on mouse input alone, but I don't see how this isn't an improvement in overall entropy, assuming the whole thing is done in an airgapped computer and so on (unlike the guy that requested that online site).