I'm just starting out cosing, so take my opinion for what it's worth.
I see that the app would fill the niche area of editing, and if it could be ported to a computer, that would be useful too. Reason is that most cosplayers like to act, and some like to act out a particular scene (as you've mentioned, a short skit) from the series they're cosplaying from. Pictures can only capture poses, but they can't capture the emotions of those within the costume to the same extent that a short video clip could.
However, too short (like Vine of 4s) wouldn't be very practical, so a recording duration of 10-15s should be fine.
After recording whatever they've wanted to, they'd then share it with their friends on social media. And the special effects could come in handy (such as typical anime-style expressions to indicate anger (red + shape), tiredness (sweat drop), etc) for comical attempts.
Why it would fill the niche area would be because I think most cosers don't want to act out a full 2 min skit and painstakingly blast hours editing in effects and subtitles later on a more powerful editing program like Adobe Premiere Pro or Sony Vegas. Stitching together pictures to form a coherent story would also be quite impractical, especially if the intended skit is short in duration.
A minor barrier to entry for larger adoption, though, would be the fact that sound recording might be an issue. The inclusion of sound into the app would be useful, but if smartphones don't have a very powerful microphone, users of the app might find themselves having to subtitle whatever dialogue is being said due to the high sensitivity or lack thereof. The alternative would be to record sound separately and slap it on top of the video (essentially a multi-track editing app), which might be more trouble than some cosers might do.
Having said that though, that hardware problem mentioned above can't really be solved by a software such as an app. I'd use an app like that if it was in existence, if only to record some funny short skits with my friends. And if a dedicated few people start producing popular short videos with the app, more might be inclined to try overcome whatever barriers they encounter, leading to widespread adoption of the app among the cosplay community in Singapore. Given the relatively large size of the cosplay community here, I think that's the best any app developer could hope for.