More than anything I hope DC doesn't see Aquaman + Shazam doing well and just sees "light hearted = people love them now that must be all we zero in on" and go full light hearted, because it isn't going to go well for them either.
The DC universe (like Marvel) is more than capable of being balanced, the only reason people leaped onto the whole dark stuff they were doing is because they chose to open it with Superman. Superman is supposed to be a beacon of light, I get it it was a novice Superman but even a novice Superman has no right playing as dark as they did with it in MoS and then they immediately jump the character into BvS which necessitates an even darker feel just because of Batman.
Superman has dark stories too, but they jumped in on something like that and completely ignored the heart of the character. Superman done well should be the heart of the Justice League, and the DC movies Superman isn't that guy.
Barry Allen is very much a character you can go dark on to begin with and then lead him into the light, by going hard on the Reverse Flash arc for him. Barry's troubles with his parents and how the Reverse Flash works into that is very gripping and dark. The 1st season of the CW TV show struck a solid balance of fun and humor while having those dark, sad moments. The scene where Barry went back and saw his mom and had to let her die in that show was heartbreaking.
The problem has never just been "our movies are too dark so people hate them". DC seems to think their characters aren't good enough to take it slow and build up, or that people won't respect their characters without severe changes that hurt the characters like Batman killing a bunch of people and Superman not really being a proper good Superman, and it's led them down this path of rushing into all their major stories and cramming films with multiple of them and trying to make the characters something they're not and it shows. The audience can feel and notice that. The reason Aquaman and Shazam work isn't because they're light hearted or less dark, but because they made the decision to do that with characters that it actually fits to do that on and they are very well done and you can tell they actually respected the characters. The audience responds to that, too. The worst part is this is only really true for the cinematic DC films, their animated films/Young Justice/Doom Patrol are all slaying it and "get it"
Marvel takes their time and just does Guardians of the Galaxy - something almost no one in the general public knew much about - and they just kill it. They had Thanos do the snap. They're going to jump into Eternals. They've slowly built their storylines and characters up over 10 years, and even launched the MCU off the back of characters that really weren't popular in the public conscience and sold them to the public anyways, and that's because Marvel goes all in on their characters and has faith in their comic work to be strong enough to play great with audiences.
It pains me a bit because I love Marvel & DC both a lot, but DC's film division just seems afraid of their own characters to perform well on their own. I don't know why.