So I'm just going to jump right into it; major spoiler alert in case you haven't seen the episode yet.
[spoiler]So first things first. I was actually pretty cool with the episode -- save for the throw-away line from Rafiki about "This happened before you were born" that threw off the timeline, but I chalk it up to him just getting old and saying stuff that isn't necessarily accurate; happens to the best of us at any age, really -- and everything that was happening up until the final confrontation. That's when it went downhill fast.
Firstly, in terms of how Zira acted throughout the episode, the toning down really became apparent when Kion started openly defying her coercion and she brought in her lionesses. Before you insert your standard paragraph on why this should've been expected, hear me out. The thing is, they were doing a pretty good job at making her genuinely frightening and threatening, or at least enough that she was still recognizable as the Zira we all know and love. To this end, the song was actually pretty good; the voice actor's delivery was appropriately powerfully ruthless and maniacal (to me anyway), a lot more than I was expecting actually, so it proved to me that Disney was fully willing and capable of selling the character and the threat she posed even within the confines of a preschool-targeting show. However, once it got to the point where she had Kion trapped in the little ravine and gave her ultimatum, she lost the intensity and just became more sniveling than aggressive. I think it was Hatari who mentioned before that even Janja has consistently sounded more genuinely threatening than Zira was towards the end of this episode, and that's very true. I really was expecting her to be on the level of Makuu and Makucha (who I've always thought was a precursor to Zira in virtually every aspect), but Disney really dropped the ball there.
Secondly, even if she was so watered down that she seemed to crumble at the first sign of open opposition from Kion, she still had a trump card to play in the technicalities of the roar. Zira knowing more about its capabilities than Kion was a great way to ratchet up the drama, and putting him in a situation in which he could actually lose it was an added extra step that I liked and wasn't expecting. But having the standard trope of the villain not understanding black-and-white morality really crippled the last scene. Even still, having Kion actually use the roar here was such an unnecessary copout; he could've just threatened to use it and Zira would've called off the attack at the last minute when she realized she was at least temporarily beaten. At that point she could back off while still retaining her villainous aura, in much the same way as the show has so far treated Makucha and Makuu, which would've left a better impression on the audience for any future appearances she'd make, because they'd know she was still Kion's most dangerous enemy. At this point she's on par with Reirei, at least as presented in this show. (Mind you, Kion hasn't actually used the roar against the jackals at this point.)
Final thing: Kion's roar blew the Outsiders into the area of the termite mounds, and the dialogue is supposed to imply that they've never lived there before, nor even seen the place. Straight-up fail on part of the show, because if they're acknowledging the events of Simba's Pride and have even explicitly said that the entirety of The Lion Guard takes place post-"My Lullaby", then there's absolutely no reason for this gag. They really just stepped in it, and it did not have to end that way; it was just an arbitrary attempt at a callback that failed harder than most of the references in Return of the Roar.
All this said, it was still among the best episodes of the series' run so far, just for everything that happens up until that certain point. Such a shame that it misfired when it did and punches were pulled where they were, and I stress that none of the problems that I took issue with are even predicated on the fact that this is still ultimately a preschool show; it was just sloppy writing and execution. And again, I'm saying all of this having accepted that the harder elements of the franchise (accurate fight 'choreography' (lions don't headbutt; no clue why they constantly opt for that move), moral grey areas, etc.) are not really going to be a thing that The Lion Guard is going to try and tackle.[/spoiler]