Speculations What did Oda mean by this?

Luffy trusts Nami the most with his Straw Hat because she‘s the one who fixes it whenever it gets damaged in some way. She’s also a Straw Hat so this works as well.

The heart is just reflecting her personality and gives a sense of warmth when looking at the drawing. It’s a soft touch that helps convey the feeling that it’s trying to portray.