There's usually a few foreign cosplayers, but most of the foreigners are there for the event proper and there's usually some foreign press. The event itself is a "garage kit (amateur figurine/model) convention, but there is a commercial section where major companies announce new releases and such which attracts international journalists. That and the amateur figures are usually much higher quality than the professional ones so people who run Japan blogs and such show up.
Since the spirit of Wonder Festival is amateur craftsmanship, cosplay fits in perfectly.
Japan Cure Cosplay Profile
Comiket83: 29-31 December 2012, Tokyo Japan
Wonder Festival Winter: 10 February 2013, Chiba Japan