“However, one who sins out of ignorance or temptation is in the category of amecha and it is forbidden to speak loshon hora about him” (Chofetz Chaim, A Lesson A day by Rav Shimon Finkelman and Rav Yitzchak Berkowitz, Day 14).
Nearly all (if not all) not-Frum Jews today fall under this category.
Many, many frum Jews do as well, so let’s not point fingers at people who are seemingly lax in hilchos tznius and hilchos lasho hara.
Also, there are specific halachic definitions of: frum, apikores, etc. –
we don’t arbitrarily define these categories. I don’t remember enough on the topic to quote sources but the definition of a frum Jew is based on shmiras shabbos, not on skirt length.