Well hyenas
are pretty damn useful, for reasons already stated by others above, but there's actually one particularly critical bit of context that's been missed here...
Has Harley been toned down from Injustice 1? Arguably so. But many other veterans have been toned down
even more (not so much the likes of Superman and Batman, lol), and the newcomers have been balanced around that slightly 'toned down' benchmark, which makes Harley Quinn more viable
in comparison to the rest of the cast.
Probably Harley's best asset is how much less meter dependent she is than so many other characters. While many need at least one bar of meter to be threatening, Harley's meterless damage is so good that she can be threatening with no meter at all, which means you'll have plenty to spare for the
many, many other choices meter gives you in this game.
And let's take a moment to appreciate how huge that is. It means you'll usually have at least one bar to pushblock rushdown and CQC characters with. Alternatively, you'll have at least one bar to MB roll with against characters who have better zoning than you. Alternatively alternatively, the threat of TS 1 into b3 may just force people's hand and get them to air tech, forcing them to blow two bars of meter
and put themselves into the range Harley probably wants them to be, just to avoid having to take Harley's stellar meterless combo damage + maybe get put into a hyena set-up too.
At the risk of sounding like a fool months down the line, I think Harley is, comparatively, in a better spot in Injustice 2 than she was in Injustice 1. Still not top 5 though.