The dialog is as follows
Sheldon: How do you propose we move forward?
Howard: Uh, for starters, we could stop insulting each other.
Sheldon: That’s a great idea. And in the spirit of our new arrangement, I will mask my surprise that you came up with it.
I kept staring at the bold sentence for quite some time and looked up examples of usage of "mask" and still don't get it.
Sheldon is surprised that Howard came up with that idea, but he is hiding (=masking) his surprise with Howard's action of coming up with that idea? I don't get it. How is that supposed to be funny?