I'm actually doing that right now. I found my next cosplay through fanart, and only started reading the manga because I don't want to look like a moron or be unable to correctly interact with other cosplayers of the series (if there are any to be found). I dislike almost everything about Magi except for probably three semi-minor characters and an antagonist. I think it's worth it, though.
I'd suggest researching your character and learning about them rather than reading the entire comic. I personally find Homestuck dull and confusing (not to mention the issues with Earthbound/Homestuck), but the cosplayers and fandom are pretty...robust. Interesting. You know what I mean.
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COSPLAYS IN PROGRESS
Utatane Piko - default (Vocaloid) - 90%
Connie (Shingeki no Kyojin) - 0%
Sheik (OoT/SSBB) - 53%