Minicel foam in thin sections is about as limp as a wet noodle. OK, not quite, but you get the point. Additionally, a cored laminate derives it's stiffness from the separation of the inner and outer skin. Minicel compresses under stress, so your laminate (assuming you could actually make one) would become more flexible just when it needs to be stiff. This flexibility would result in cracking of the resin/glass matrix. And to top it off, Minicel is polyethylene foam, which is relatively hard to bond to. Bottom line: It would be a really poor choice as a core material.