thats fin if your using tiny tiny wireing like yours (your amp must be either very small or very under-powered). Actually even then its not ok, that cabling is obviously ment to take the load of something else, and with your amp + the original load it could cause problems!
I would suggest direct connection to your battery every time. either use an existing hole or make a new one (make sure you pait it and seal it so it dosnt rust or cut the cable).