There's no definitive answer to this question - it depends on the girl in question and what sort of relationship you have with her. If she's a close friend or family member, then writing a song for her could be seen as a thoughtful gesture. However, if you're not particularly close to her or if she's in a relationship with someone else, it could come across as creepy or stalker-ish. Ultimately, it's up to you to gauge whether or not writing a song for her would be viewed as romantic. If in doubt, err on the side of caution and don't do it!