On a mobile, does using a password instead of a PIN make encryption stronger?
What key does the phone use for its encryption? Would it use the PIN itself?
It depends entirely on the phone. But in general, using a strong password is always better than a weak pin.
The phone may use a key derived from the pin for encryption, in which case pssword is much better and honestly quite necessary.
It may use some secure hw chip to store the key and the chip may limit number of attempts. In this case PIN may be enough.
It can also use something else.