Unpopular opinion, but I really like Mega Lucario Z's design. This is mainly due to how it contrasts with regular Mega Lucario.
Whereas Mega Lucario grows additional spikes all over its body for offensive purposes, Mega Lucario Z turns those spikes into steel plating for defensive purposes. It conveys that Mega Lucario Z is more defensive than offensive.
While Mega Lucario can concentrate its huge aura into a destructive force, Mega Lucario Z surrounds its entire body with its aura, cloaking itself to increase its defense, flexibility, and agility. Mega Lucario Z can use its aura in a variety of different ways, such as stretching the aura thin into a shield.
It's new design (mainly coloration) also emphasizes its Steel type more than its Fighting type, which is what Mega Lucario did. Mega Lucario gained brown/red accents, additional spikes, and its dreads changed to look more like a headband. This all screams "Fighting type."
Now, Mega Lucario Z turns its spikes into steel plating and is much more teal and gray rather than navy blue, black, and brown/red. Its dreads are much more smooth compared to Mega Lucario's rough/flowing dreads. Also, its tail is free from its chest fur.
One final thing is Mega Lucario Z's eye color. Whenever a Lucario is using aura, its eyes glow blue (at least, in the anime). Due to it constantly surrounding itself with aura like a shield, it would make sense for its eyes to be constantly blue.
by Ecla1r_