It is free to mint?
Every one can mint NFTs in Mixin at very low cost, but it's not free.
All NFTs are minted from a MTG(Mixin Trusted Group), which is a decentralized network launched and maintained by multiple parties from Mixin Community.
It will cost 0.001 XIN to mint a NFT. The fee is charged and decided by the MTG.
See also how to mint NFT.