If Doc put up a Dominic Deegan strip in place of the Whiteboard, that'd be copyright infringment. If he 'shopped in Spark in place of the Kitty, that would be infringement. If he drew Spark in, that might be infringement (but could be construed as parody or fair use, depending on circumstances). With his own character, in appropriate circumstances, even with the yell, it is more like homage or parody. Anyway the cry/concept of "Death from above" is hardly unique or originated by DD, so Mookie wouldn't be able to claim copyright on it anyway, even if his strip features it frequently.