If there's any testimony to the fact that Madoka seems to have caught on that there's really some warmth and goodness in that heart underneath the terrifying exterior, or that she's actively choosing to trust the man, it's this fact.

The girl actually gains a tiny hint of something more cheeky in her smile. But it's very light, playful. Madoka would never dare push a joke, after all. But around the people she's comfortable with? She can at least pull off a slightly playful joke like that.

So she just says: "I'm going to make sure to tell everyone how nice you are, mister."

There's no real threat behind it. Madoka would never. Granted, if anyone would bring up Yondu to her, she'd for sure let them know her opinion of him, but that's more natural! But it's not like she's going to seek out everyone for this specific purpose.

It's just a playful joke, because she's decided she trusts him enough to make one around him. It's even accompanied by a very slight (adorable) giggle.

"You better watch out..!"