Tragic heroes are usual slightly better than ordinary men and have an excess of a quality--may be positive or negative--which causes them to make choices that bring about their downfall and the downfall of the people around them.
Often this quality is pride (hubris); this is not Winston's downfall.
Winston has two issues: he wishes to be part of a revolutionary movement and he wishes to speak the truth, as he sees it. Both of these desires leave him vulnerable. Making the wrong friends and trusting the wrong people leads to his arrest, along with the arrest and eventual destruction of Julia; trying to speak the truth as he sees it leads to his mental breakdown and his ultimate destruction.
Yes, he is, in my opinion, but it is sometimes difficult to see this because Winston is such a self-centered character, and thus a problematic protagonist.
He is motivated by his own agendas rather than any altrusim. However, is bitterness is understandable, given the stranglehold of the ruling party.
As a character, Winston is tragic because he eventually loses his desire to fight the party. They, in effect, "win."
To my opinion as I stated elsewhere on this site but probably also on another, Winston wasn't an hero, not 'an unlikely one', not a tragic one, he also never had any ambition to become a hero, to my opinion he was a nice friendly guy who wanted change for (his) society for the better, but of course utterly failed because of the problem he had to experience with the rats with whom he was treathened that they should walk and bite on his face, that was just his breaking point, and also as I come to more and more thinking about it, should that society have changed for the better as he had allowed the rats to come over his face, more than probably not, they should have just come and walk and bite over his face, I'm saying this because someone elsewhere suggested that his society should have changed than, however I do not allow myself to believe that!