Most of the outstanding token supply is still held by the Stellar Foundation to be distributed. In my books, that means that it's still pretty centralized, but much less so than other ICOs or early coins that seem like outright scams.
The most charitable interpretation is that Stellar is on the right path to decentralization.
"Although there have been many attempts to corner markets by massive purchases in everything from tin to cattle, to date very few of these attempts have ever succeeded; instead, most of these attempted corners have tended to break themselves spontaneously."
Whether a cryptocurrency is centralized or decentralized has a specific technical meaning, namely whether a party is, by design, in control of the security or trust of the currency and can unilaterally make decisions. By that definition, Bitcoin is decentralized and Ripple is not.
If you remove the current state of the token supply, then I'd say that Stellar seems to be relatively decentralized as evidenced by the number of verified validators not under direct control or ownership by the Stellar Development Foundation (around 13% by number; don't know the validator splits by volume).
The most charitable interpretation is that Stellar is on the right path to decentralization.