I thought is was pretty obvious it was a joke, myself.
Anyhow, as to the topic itself, the show could still get darker without going beyond Disney Jr. stuff. As Lionkingview points out, it could be more psychological than gritty violence. All it would take for the series to get darker is to reveal more about Scar's past, or do something else like Never Roar Again but more regularly. Or the Outlanders having whatever conflicts they have with the Pridelanders and the hyenas could make things darker, too. No one has to die or get into intense fight scenes or anything, though a loss through death by natural causes is a possibility in the future for this show considering we've already seen a funeral (though I wouldn't expect a major character to meet this fate). At any rate, there's a lot they could toy with to make it darker and still keep whatever it is toned down enough so as not to upset parents or drop too much on young kids.