It's in the rule book. Page 364, paragraph 5: "All appearances by Marx must include at least one laser mouthing."
I'm having trouble figuring out whether these Shadow Marxes are his normal boss sprite or his Soul sprite. I do seem to recall that most of the time when Marx appears in this comic, it's his Soul sprite...
@Jarkes: They're the normal sprites.
The soul sprites have the spiky/fluffy pfoofs on the points of the hat.
And also the soul sprite is a lot crazier-looking than this.
@Luigi_96: Right, I should've realized that.
...God, the Marx Soul death sequence and scream is probably even more horrifying than Zero and 02. The Kirby games sure seem to love their super-dark final bosses. Though apparently the final boss of Triple Deluxe is different from your typical Kirby final boss...
@Jarkes: Honestly, I loved that death sequence. It's a pretty good reward for finally beating the True Arena.
Also, I would very much enjoy using KSSU's sound track 347 to wake someone up.
(... I am an evil person.)
@Luigi_96: I've done that before... Not the greatest reaction... It was hilarious though
That is surprisingly evil from you. But it does sound hilarious in theory. XD