There's been a huge push for more diverse characters in stories. *Gasp* There are more types of people in the world than white, blond people--who'd've thunk it! It may be easier to write that because it's what you know (if you've seen my picture you'll know I'm a white, blonde person). But you don't go to school with people of one race or hair color. You don't work with people of one race or hair color. Your friends aren't all the same race with the same hair color. I think you get what I'm saying. Adding people from different races or cultures has been intimidating for me--more because I don't want to ignorantly offend someone. In the tradition of paranoid writers everywhere, I'm not going to tell you what ethnicity my new MC is--I don't want you to steal my idea. The very most important thing you can do when writing a character of a race or ethnicity you're not afraid of is to find someone who is. I'm very fortunate that I have a friend who's of this ethnicity and she's even picked the brains of her family members for me.
The internet is a huge, massive resource for writers. There are websites dedicated to certain ethnic groups with common phrases and cultural breakdowns. Get on message boards, reddit or twitter and find someone who's willing to help you learn about a group of people different than yourself.
Is it bad to write about blond, white people--no! But when the community is inundated with stories about the same types of people, they start to run together and can become boring. I'll start envisioning Jennifer Lawrence as all the main characters (I may read your character as looking like Dakota Fanning, but my mind always pictures someone else--I'm just strange that way).
Perhaps the best way to get more diverse characters is to encourage more diverse writers, but there's nothing stopping me from learning and writing about a culture or race I'm unfamiliar with. And just remember, any mistakes or stereotypes are completely my own fault--and remember, I'm human and I'm learning.