How would feel if I told you I realized I didn't know how to pronounce her last name?
It's normal if the name is hard to pronounce. Do you know how it should be pronounced?
Perhaps I should write her last name as 'bla-dah'
It's okay if you don't know her last name, but it is disrespectful to not make an effort to learn it. Ask her what her last name is and use it properly moving forward.
Fuck. I always make fun of her for her name behind her back, saying it is stupid. I guess I'll learn it.
Yeah, it's not cool to bully someone with their name. She probably feels very disrespected by you for not learning how to pronounce her name correctly. I guess it would be smart to apologize to her and learn how to say her name correctly.