Nope, Actually ethereum is an altcoin or alternative cryptocurrency and it is the substitute decentralized than bitcoin , Because ethereum sometimes the price of it getting lower or more than of it is than bitcoin that is really a decentralized that can more efficient than ethereum.
An altcoin can be more or less decentralized than Bitcoin. It doesn't matter that it's not the first and the most popular cryptocurrency. I don't know how you came up with that.
Also, altcoins arent substitutes of Bitcoin. They are alternatives which means different variants.