He is in my view. Boba although a clone of Jango, was a kid partly by Jango (until his demise). So he can be seen as an adopted foundling. Now the question is Jango a Mando. If non film canon is accepted, through the magazine Star Wars Insider, an issue with an article on Mandalorian history, backed up with the chapter on Mandalorian history in the book the Bounty Hunters Code, states that Fett was a key player in its history.