Duolingo regularly changes their app icon to weird things. It's not done to get users to buy things. It's so you post the icon on social media as a form a free advertising. The company really loves that their mascot has become a meme and play into it.
The Spanish or Vanish ad is terrifying. I don't know how the marketing team got permission to have "I love the taste of human flesh" in Spanish as one of the lyrics in the song.
This scared my housemate too. It's manipulation to get you to continue, so you buy their subscriptions and/or microtransactions. I love Duolingo, but fuck this manipulation shit.
It's a [accidentally(?) nightmare-fuel] sad face designed to get you back in so you spend money. It might not be the worst thing ever, but it's still pretty gross. Also the app is used by children.
Emotional manipulation wouldn't be a good way, especially not a face that scares the shit out of people (I can't even look at it, as I'm sensitive to uncanny valley and distorted faces, especially the kinds of sad or scared faces 1940s cartoons used to freak you out. My nightmares often have these in them). Especially not so that it can get you back to spending money, and tripley-especially not so that it can get you back to spending microtransactions.
Same. The bird is dying because you haven't completed a lesson in a long time. He so sad... only you can cheer him up.
Edit: I just completed three lessons and he still looks haggard, so I don't know what that little fucker wants. Probably for us all to purchase the paid version.