A few things:
1.) Both TDK and Iron Man were extremely influential to pop culture and film. The Dark Knight was more influential to blockbusters and filmmaking as many films have tried to replicate the tone than Iron Man but also The Avengers started the wave of cinematic universes that most of which failed (Dark Universe, Transformers Universe, ASM universe) or did well but never captured the same results just yet (DCEU, Monsterverse, LEGO Universe, XCU). The MCU has succeeded with popcorn fare to pretty great blockbusters although only of few of them imo reach nirvana of phenomenal CBMs or blockbusters for that matter.
2.) Dark tone didn’t hurt the DCEU, bad writing and divisiveness did. Now before I’m deemed a “Marvel Zombie”, I love WW and SS, and like MOS which admittedly grew on me and the BVS Extended version is alright not to mention outside of BVS and Sucker Punch, Zach Snyder does good work (Watchmen is one of my favorite CBMs fight me but don’t @ me). However, both BvS (theatrical) and JL sucked. The writing and character development made me dislike BvS the most and JL was so desperate to be the Avengers it became something so soulless and mundane it almost felt executive made. WB was so worried not to have another BvS they screwed SS and JL to make them lighter even though (tbh JL Snyder or Whedon would’ve been screwed BO wise as it was a direct sequel to BvS) they forgot story and writing was key.