Define best but it's a pretty reasonable approach. Some DNS servers (powerdns is an example from memory) use mysql over more traditional back ends like dbm which aren't so hot for high volumes of zone changes. I imagine a site like wix could be pretty tough on DNS.

Re: nosql, I'm coming at that with really positive experiences in Cassandra but I can't imagine what kind of DNS system it would be a good fit for. The ability to tune CAP to fit DNS may be useful but in general I think of Cassandra as the solution you think about when your 2 node vertically scaled monster can't keep up.