I don't know how that plays out with mansions though. Whether a mansion is worth $30M or $10M is often hard to predict with the pool of potential buyers being so low.
But yes, a tax on "unrealized gains" basically amounts to a property tax, not anything related to an income tax.
The main difference being that a property tax only takes into account the assessed value and ignores what you paid for it. They tax the value, not just unrealized gains.