I like to turn bone-in skin on chicken thighs halfway through the cooking time to help the skin get crispier. You can also smoke them skin side down to render the skin as much as possible. However, it will never be as crispy as cooking at a higher temperature. You can also remove the chicken thighs early from the pellet grill and 'reverse sear' with the skin side down in a skillet to help the skin crisp up.