var designs = {
    shirt : ['Simple', 'V-Neck', 'Polo'],
    hoodie : ['Sweater', 'Hoodie'],
}

function chooseType(type) {
    var btnGroup = document.getElementById('btn-group');
    var btn = document.getElementById('design-btn');

    for (k in designs){
        if (type == k) {
            for (i=0; i < designs[k].length; i++) {
                var clone = btn.cloneNode(true);
                btnGroup.appendChild(clone);
                clone.classList.remove('d-none');
                clone.innerHTML = designs[k][i];
            }
        }
    }
}

<div id="design" class="d-none">
    <p>Design da peça:</p>
    <div class="btn-group" role="group" id="btn-group">
        <button onclick="chooseDesign()" type="button" class="d-none" id="design-btn"></button>
    </div>
</div>