I ask because I've literally heard it both ways and Google turns up results that have both basic narratives. I've seen things that said the majority of victims of the Holocaust were basically secular Jews who were largely indistinguishable from their neighbors until one day a government decided they weren't. I've also seen it said that the majority of Jews killed in the Holocaust were seriously religiously committed Orthodox Jews, who were highly visible to the Nazis, such as the people in Talmudic academies in Vilna. Which narrative is closer to what we actually know?
It's really not much about narratives, so much as it's about looking at the demographic information of the places where Jews were murdered and how many people were murdered in those places. About half of the Jews murdered were from Poland (2.7-3 million- 85-90% of Poland's Jewish population), which had a very large Orthodox community but also non-Orthodox Jews. According to Nechama Tec, only about 10% of Polish Jews were assimilated. Of course, the Nazis didn't distinguish. From this perspective, and given the fact that, for example, another 300,000 Jews were killed in Hungary, where 35% of Jews (EDITED- maybe even up to 50%) were Orthodox, plus the many Orthodox victims in Lithuania (which, like Poland, had an incredibly high death rate- between 85-95%- and also a large percentage of Orthodox Jews in its population of 200,000 or so Jews) and any smaller Orthodox communities in other affected countries, it's probably fair to say that strictly numerically, a majority of Jewish victims were Orthodox. That didn't mean they were all "ultra-Orthodox," who would have been targeted by their dress- Jews were targeted because they were identified as Jewish to the government, and the Nazis had access to the records. That affected Jews who looked super religious and Jews who looked exactly like their neighbors. The fact that victims were probably numerically majority Orthodox has basically nothing to do with how identifiable they were and more to do with other factors, which are numerous and speculative and honestly in some ways random (ie. the fact that the country with all of the death camps was also the country with a massive number of Orthodox Jews probably had an impact).
Sources:
Tec, Nechama, When Light Pierced the Darkness
USHMM, Jewish Losses During The Holocaust: By Country
YIVO Encyclopedia: Hungary: Hungary from 1918-1945 (note- Status Quo Ante includes those practicing in the Orthodox tradition, even if they did not consider themselves part of the official state-sponsored Orthodox community) EDITED: The Encyclopedia Judaica seems to indicate that the Orthodox and Status Quo Ante communities could have actually been 50% of the Hungarian Jewish population!
(Whew, this was surprisingly fun...)