Salamualaikum,
i dont think it's the fact that not wearing hijab makes a woman a bad muslim, but it's the fact to think: who are you to label others as bad muslims? which madrasah did you study at? which shuyukh gave you ijaza? my point is, it's not that they aren't being good muslims, it's that we can't think ourselves as higher than them because our external is closer to the sunnah than theirs. their actions could be much better than ours.
This reminds me of a story i heard once in a lecture. it goes like this: there was this boy around 12 years old (true story), and he ended up getting into drugs. so, one time he was smoking with a muslim. so, the boy got into asking the muslim about Islam. as a result, the boy ended up converting and attending a madrasah full time. now, whenever the boy (i forgot his name) tells the story of how he became muslim, he would tell other people not to curse the muslim that was smoking with him, because whatever the boy does will be recorded for the muslim. whatever sallah, zakah, hajj, sawm, etc. that he boy does will be on the smoking muslim's record. even though obviously to smoke is a bad decision, it's the cursing them out part and thinking of yourself higher to label them as "bad muslims" which is the conflict, not the fact that they're sinning.
wAllahualam. Salamualaikum.
, akhi I agree with you. how can one be a good muslim if one knowingly goes against Allahs command and the examples set by the Prophet, his wives, family, and companians? Its just not possible. 
