Naoto's character development was about accepting her femininity so the changes to her appearance like her growing out her hair and not taping down her breasts anymore made sense. Shiki's development centered around her getting over her jealousy of Eri and loving herself for who she was. Her suddenly completely changing her appearance(remember, she looks like
this), wearing headphones, which were a symbol of isolation in TWEWY(and those aren't even the same headphones that Neku wore either), and still having Mr. Mew, who she lets go of in the ending doesn't exactly strike me as the results of that character development. Much more like massive character derailment if this turns out to be Shiki.