When the MCU wants to they can do a serious fight hence Iron Man vs Captain America I dont recall a single joke being used when they were fighting in Civil War
People love blaming all of Justice League on Whedon to act as if Zack Snyder was gonna deliver something special like give me a damn break.
Also BvS wasn't a box office faulure in any sense but a movie with first cinematic appearances of several iconic a-list superheros and didn't even reach 900M overall