It’s unknown whether Jango (and therefore also Boba) were actually Mandalorian. Jango claimed to be from Concord Dawn, the planet where all mandalorian warriors were exiled when their society turned pacifist, but the government of Mandalore regarded him as a “renegade and pretender”. That doesn’t necessarily mean that he wasn’t mandalorian by birth though, just that the government didn’t like him calling himself Mandalorian.
The ‘New Mandalore’ and their pacifist regime cast out all of their warriors some time before the prequel era, and believed them to have died out some time later, so a man with a reputation as a ruthless killer emerging and calling himself Mandalorian wouldn’t sit very well with them as they try to maintain their image as a peaceful people.
I believe that Jango and Boba are true Mandalorians by birth, but the Mandalorian government just didn’t want them tarnishing their image.
Also, while we’re on the subject, whatever Boba is, every clone is as well. If Boba is to be considered Mandalorian, then all clones are Mandalorian too, genetically at least.