Both machines are treated like they're garbage in the presence of the 'amazing double zeta'.

The reason they're treated as garbage is because during the Gryps Conflict and the First Neo Zeon War the Titans and Neo Zeon's mobile suit development was absolutely crazy, they had access to more funds compared to the AUEG and in turn could churn out better and better Mobile Suits in like a week. The Double Zeta is considered so powerful because of its 3 core boosters acting as 3 separate power sources for the ZZ's Double Beam Cannons and Beam Claymore's and Hyper Mega Cannon.

I absolutely love the MKII and the Zeta Gundam is my favorite Gundam in any of the shows, however they simply couldn't keep up with the new developments that Neo Zeon could just pay for with their insane amounts of Gold they had.