Per this article, Suicide Squad suffered from a tight writing schedule and (especially after Batman v Superman serious worries by Warner Bros. that the film being made was going to be too dark (and not in keeping with the much more antic trailer that seemed to be so popular).
In the end, the studio forced through a fair number of changes and reshoots to lighten things up. Ironically, I think things would have been better if they hadn't tried to compromise between the two perspectives — either a much darker movie would have been better and more interesting, or something that was consistent with (and structurally built around) a lighter tone would have been more successful. Suicide Squad tries to split the difference, and feels a bit muddled as a result.
‘Suicide Squad’s’ Secret Drama: Rushed Production, Competing Cuts, High Anxiety
The upheaval like that behind Warner Bros.’ DC team-up is becoming a staple of studio franchise filmmaking.
I had worried about this as soon as the rumors started floating around and I'm surprised to hear from a few others that the movie isn't a complete wreck. It's hard to turn a grim dark movie into a lighthearted adventure, but Suicide Squad apparently came this close to pulling it off. Still, it would've been better to pick a single vision and stick to it. I imagine that was hard after the one-two punch of Deadpool's success and BvS's bashing.
+Marty S. Exactly. It didn't quite pull it off (actually, fell appreciably short), but also got out of trainwreck material. It's simply a film that was passably entertaining, has some controversy, had some laughs, had some groans, had some scenes that people will remember, and will do nothing to get DC's movie franchise out of the hole.