It's not just you—T-Mobile sends a lot of junk mail, whether or not you are a customer. Luckily, it's fairly easy to remove yourself from marketing communications from the phone service.
Simply head to t-mobile.com/opt-out/marketing and select "Direct Mail." You may also choose to opt out of email, phone calls, and text messages from this form.
From there, the website will ask for your address. Complete the form and hit submit. Like most companies, T-Mobile prepares their mailers well in advance, so it may take a few months for you to notice a decline in junk mail.