Think of it this way: they don't know your dad as much as you do, so when they say that, they clearly refer to the "surface" part of your dad (I'm guessing the one that looks good?). Take it for what is it: an (ignorant) compliment. If you can't see it this way, then.. just ignore these comments, I guess. :/ Of course they don't know what you've been through, so their opinion regards this doesn't really matter.

Besides this... I'm really sorry to hear that about your dad. Those comments are extremely mean-spirited and you don't deserve them at all. Please, don't listen to a word of what he says - you're really strong, you're still standing, fightning and trying to improve your life. If these aren't signs of a strong person, then I don't know what they are.