I don’t think someone is frum just because they dress modestly. I have seen non-Jews dressing modestly, covered from top of the neck to their ankles, in loose clothing, dark colors,very plainly.
People are frum if they act with honor and dignity when they follow the Torah. It also helps if they are not selective in their observance (like dressing b’tznius, but committing monetary fraud, while clearly identifiably Orthodox Jews).