Well, first of all, they’re wrong. Darling in the Franxx is anything but average and dull (for better and worse).
The main thing to keep in mind when watching it is that the show isn’t about the mecha fights. Sure, they’re some of the best mecha fights this side of Gurren Lagann, but the main focus of the show is always going to be on the relationships in Squad 13, and Hiro and Zero Two in particular.
At the end of the day, the mecha fights are window dressing for one of the best character dramas I’ve seen in years. Some top notch window dressing, but still window dressing.