Can't wait to see the rest, under the helmet...
Your decal is suffering from the 'silvering' problem, i.e. there is a space between the film and the helmet painted surface. This happens in general when the surface is matt, so the film doesn't conform well to the surface. The results is that we can tell there is decal. We can see its outlines clearly. The best would be to start over, and coat the surface with a gloss (or semi-gloss) varnish before the new decal. If no spare decal, trim out the existing film until the white cross, and seal what remains with a layer of matt varnish (for protection).
Good luck,
Patrick