No, it's absolutely not bad writing in any defensible technical definition of the matter. Clearly it's not the way you'd write the scene but there's a huge difference between "not the way I'd do it" and "bad." It is poor form as a critic not to acknowledge this difference.
The change of hair may be a visual flourish added in post-production, so the script doesn't acknowledge it because Ankh's human look hadn't been designed yet.
The script may not want to make a big deal out of the hair change because Ankh changing the cop's hair color and style is honestly a very silly and unrealistic flourish, the sort of thing that may look good but doesn't necessarily need to be lampshaded.
The script may just not be interested in subjecting the audience of an action-comedy to a conversation, however brief, about the hairstyle of a male character that said all of about 5-6 lines before he became Ankh. A lot of viewers just won't give a **** about the hair thing no matter what.
Finally, Hina is a supporting player and it's not necessarily inappropriate for the script to gloss over her plots in favor of other things-- action scenes, comic relief, exposition, Eiji and Ankh doing basically anything. Hina is probably going to matter very little to OOO's story in the long run and, frankly, there's only so much screen time that she deserves.
(Note: I haven't seen the second episode yet, so I don't know what was put onscreen instead of her making a hairstyle comment. But I bet most of it was vastly more interesting than some chick talking about her brother bleaching his hair and getting a perm.)
