The DIC dub did call the Princess "Serenity" at times, suggesting that "Serena" was a nickname, perhaps to distinguish her from her same-named mother.
Luna only knew they were looking for a "Moon Princess," not a "Princess Serena." If you have no problem with Japanese!Luna and the rest of the gang not remembering that that Moon Princess looked exactly like Usagi, or realizing that Usagi Tsukino a.k.a. Sailor Moon might be the "Tsuki no Purinsessu / Moon Princess," criticizing DIC!Luna for not recognizing Serena as Princess Serena (which, again, is a nonissue) is hypocritical.