Mass attack also had nice anime references, yet anime lore and game lore will continue to be two separate things, both enjoyable in their own right.
About theories on Marx, I don't know if there might have been an official explanation but he could have had those crystalline bat wings from the start and only revealed them for the big fight. He can do that in Mo~Pupu! by the way, yet I know the manga isn't canon. These are my thoughts: Marx makes Kirby summon the Galactic Nova and wishes for the control over Popstar. If the bat wings were the fulfillment of that wish, Nova would not have to fly over to Pop Star to attack it, I guess. Kirby chases after and destroys Nova to prevent this, then gets attacked by an angry Marx. Because he shoots lasers and creates black holes, I guess Nova did give him some more power (otherwise Marx could have easily attempted to take over Dreamland way earlier), yet none of this magic is in need of wings. He spits a lot of attacks like ice and lasers, kinda gross. Marx Soul also has very similar wings despite Nova being in no condition to create them. All Marx happened to do was absorb some of its remaining power. Besides, I have doubts about Nova being able to create life
I know it could very well be different. It could just be the other way around and Marx can spawn his wings only by making use of powerful magic given to him. That would make a nice explanation for their strange looks. A character with very similar wings, Flandere Scarlet from Touhou, is also said to have artificially created them herself. The color, hearts and arrows could be traced back to Nova's color, heart and weathercock and like Marx does. Yet Nova seems unlikely to have created them itself as they are pretty useless. Any other magical boss can fly without them (even no-limbs-at-all-giant-flying-eye-blob Zero while bleeding).
Although the artificial wings scenario sounds more likely, I personally love the possibility of Marx having limbs. Or do you want him to be some kind of evil Noddy clown puffball? Maybe leading such existence is (part of) his motivation?
Feel free to dismiss my opinion on Marx as it is only the over-the-top bad-ass-hole Marx I like who has hands and magic and can transform by free will, the absolute madman.
Talking about Mo~Pupu!, Marx gets to stay in Dreamland despite his attempt of destroying it because the townspeople love a good fight now and then. Kirby tells him it is great 'fun battling Marx' and wishes to 'play with him again some day'.
I.e. Marx would not even have to be reformed much to be included in some more games, he could show up as a boss or ally not advised to be trusted and attacking as a test, sort of like Meta Knight does.
Tldr: I prefer a competent Marx.