Liz really does not look nasty to me in the last panel. More like, "What's wrong with her?" as if she honestly can't understand why April is on edge, or why a 26YO calling her teenage sister "Picky Face" would get an angry reaction. She must honestly think that April was supposed to smile and say, "You're right; I am a picky face! Thanks for helping me out by suggesting toast!"
Also, here's a interview with LJ.