That's a tough one. Obviously you don't want to step on any toes, but you're right that you deserve to contribute to the tone of the decor in a place you share.
Let me ask you this: you don't like your new roomie's existing furniture so do you currently own anything that could replace it? If so, it's a pretty easy ask. You can simply say you'd like to incorporate some of your own favorite pieces into the apartment's decor and would she mind making room by moving a few of her things into her bedroom? Not unreasonable at all.
You happen to currently be on a site that has lots of beautiful examples of simple, more minimal rooms. See that search box up top? Use it. Find some examples you love. The next time you two are hanging out, casually show her a couple of them and mention that you've always loved the clean look and why don't you guys try to pare down a little bit.
Feel her out. Maybe she simply hasn't really noticed the clutter accumulating (as tends to happen) and would welcome some wrangling in that area. She might love your help in selling some extra items on Craigslist to make a few bucks.
Remember, if you really want to change the look/vibe you'd be well served to invest in a few things that are you style to tip the balance a little more in your favor.