Does having front bangs which cover your forehead cause accutane to be less effective because build up of sweat? i broke out on forehead on last month

Yes: Keeping bangs short is part of basic hygiene changes to reduce severity of acne. Accutane is usually started if all these efforts as well as friendlier medical options fail. It's all a matter of what's most important to you, and if you're willing to endure worsened acne because you love your bangs, then that's ok. But, just be aware losing the bangs would likely improve forehead acne.