If a WinRT class can be created from C#, it means it's activatable (or it's associated with another "statics" class which is), which you can check if you go to NotificationData documentation:

[Windows.Foundation.Metadata.Activatable(262144, "Windows.Foundation.UniversalApiContract")]
[Windows.Foundation.Metadata.Activatable(typeof(Windows.UI.Notifications.INotificationDataFactory), 262144, "Windows.Foundation.UniversalApiContract")]
[Windows.Foundation.Metadata.ContractVersion(typeof(Windows.Foundation.UniversalApiContract), 262144)]
[Windows.Foundation.Metadata.MarshalingBehavior(Windows.Foundation.Metadata.MarshalingType.Agile)]
[Windows.Foundation.Metadata.Threading(Windows.Foundation.Metadata.ThreadingModel.Both)]
public sealed class NotificationData
{...}

If it's activatable, then you just have to find it's activation class id, and call RoActivateInstance . The class id is visible in the corresponding .h file (here windows.ui.notifications.h) and it's usually very straightforward: <namespace> + '.' + <class name>:

...
extern const __declspec(selectany) _Null_terminated_ WCHAR RuntimeClass_Windows_UI_Notifications_NotificationData[] = L"Windows.UI.Notifications.NotificationData";
...

So here is how you can create it:

HRESULT CreateNotificationData(INotificationData** data)
{
    if (!data)
        return E_INVALIDARG;

    *data = nullptr;
    IInspectable* instance;
    auto hr = RoActivateInstance(HStringReference(RuntimeClass_Windows_UI_Notifications_NotificationData).Get(), &instance);
    if (FAILED(hr))
        return hr;

    hr = instance->QueryInterface(data);
    instance->Release();
    return hr;
}

PS: using C++/WinRT is usually easier to use than ATL, WRL or WIL for WinRT types.