That is what we do. Always travel with our coffee mugs. No spilling, keeps warm, and easier to bring back to the cabin. And of course doing the same as you...fill the paper cup, then pour in to mug.
Had to stop one crew member on EX that was trying to "help" while he was setting up in the CL. He grabbed my mug to fill it for me, but I told him about the paper cup transfer. His answer..."Oh it's Ok, doesn't matter". Ughh...