It's not direct, but...
I think the most natural way of expressing this is through a couple of idioms:
"It's what's on the inside that counts"
and
"Beauty is only skin deep."
Although they're not literal translations, but they sound more native to me. To say that "what matters is the heart," in English, sounds a bit... sappy, childish... like what you would find in a Disney movie. At least, it sounds that way to me (even though I speak like this all the time, I don't think it's how you want to sound...you want to be taken seriously). If someone disagrees, please tell me! I'm probably wrong :sorry: