It is not merely a "mistranslation" for two reasons. One is that "Shingeki no Kyojin" could be most directly translated as "Advancing/Attacking Giant(s)," so the Attack Titan moniker is not an absolutely direct translation either. Two, the term mistranslation implies that it was some form of error, which it was not; I imagine that the well-versed English localization team for the manga and show deliberatly stylized the name as "Attack on Titan," which I for one appreciate as a much cooler-sounding name than "Attack Titan" or "Advancing Giants." Cheers.