Using ad blockers is piracy, insofar as you’re avoiding paying the price the content provider has set for that content. The price is watching the ads, rather than being something directly monetary, and you’re not paying it.
That said, neither that nor piracy are theft, and in both cases I gladly pirate because the prices in most instances have gotten away too high for what you get. Either in terms of subscription cost, or the time and quantity of ads delivered.