I strongly disagree about them not fitting in terms of aesthetic/art style. They're not much different than Yorick's Mist Crawlers, just a bit more ghastly-looking. Maybe they look a bit less defined, but they're also supposed to be minor Mist creatures being expelled out of her targets, so the vagueness is probably intentional.
I think if you're talking mechanically, then yes, they do stand out quite a bit compared to the background, champions, minions, or other targets. This is, in all likeliness, intentional so that both Senna and the other players can immediately identify her Mist Wraiths at a glance and make decisions revolving around them. (including, but not limited to, when to try to snag them, using them as part of her harass pattern, etc.)