You just dont know good comedy! I dont think you understood the joke, Jimmy.
Basically, in Shrek, the main character, an ogre named Shrek, has to rescue a princess with a talking donkey from a tower. Shrek, fearlessly does so, because hes not scared of a dragon (who by the way ends up becoming a couple with the talking donkey).
Now, when deadfroggy said 'Im Shrek', hes implying he is not scared of a dragon due to him being Shrek.
good one deadfroggy, keep it up!