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).
Fair point, but AFAIK there's no noticeable difference regarding security/"randomness" unless you have serious concern about good entropy source.
This might help you if you are using an 'offline' computer to generate private keys that has previously been exposed to the internet, but that will not be connected to the internet in the future. An attacker may anticipate this and mess with the /dev/random function and nothing else.
As i mentioned earlier, compromise
/dev/random requiring you to compromise the OS itself. And if your OS were compromised, most likely your Bitcoin already stolen with other way (such as keylogger, copy wallet.dat and access private memory which might contain private key if you open your wallet).