The Altcoin Season Index has fallen to 17 as of December 21, indicating a subdued sentiment in the altcoin market, according to CoinMarketCap data. This represents a significant drop from its September 20 peak of 78, with the current weekly average at 21. The index reveals that only 17 of the top 100 altcoins by market cap have outperformed Bitcoin over the past 90 days. The CMC Altcoin Season Index measures altcoin market dominance by comparing the performance of the top 100 altcoins against Bitcoin. Recent analysis suggests that altcoins continue to underperform in a market environment dominated by Bitcoin's strength.