To split hairs, I'd say that's not piracy. To me, piracy would be sharing that software (or receiving that software) to others who haven't paid for it. The security circumvention may be a necessary part of the piracy process, but I don't see that as the issue. Consider the scores of games in the past that have had draconian DRM schemes, which unnecessarily require always-on Internet connections or regular dialing home to some server -- maybe a server that no longer exists as the developers do not support the game. In these cases, modifying these impediments to the functionality without sharing the software wouldn't be piracy in my eyes.
Actually the correct analogy to ad blockers would be downloading a free trial/demo and cracking it for own use without paying for it - would you consider that piracy?
Is it piracy when crackers remove part of the software that secures it as that annoys people and they only want to use the functionality?