How a hiking jacket should fit depends on what layer it is. Base layers should fit anywhere from snugly to a bit loose, to maximize the material’s (whether its merino wool or synthetic) wicking ability. Insulation layers should fit somewhat loose, as you want the down or synthetic down equivalent to have room to puff out, but not so loose that it's easy for a draft of wind to sneak under the hem. Rain jackets should fit fairly loosely, in order to accommodate the different layers you are likely to pack underneath them.