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.
facebook page // nh/mass cosplay events
Next event: ???
COSPLAYS IN PROGRESS
Gig (Soul Nomad) - Constructing
Vaati (LoZ Minish Cap) - Planning
Soren (Fire Emblem: Radiant Dawn) - On hold