I suppose by crypto index, you mean something like the index fund (https://en.wikipedia.org/wiki/Index_fund).

I think 'crypto index' will be just like index fund in terms of performance, but the cycles will be much faster and quicker than the index fund. Actually, if we take a look at the crypto market, their historical price movement does seem to be mimicking or similar to how traditional (gold, silver, stocks) assets performance. Will it be good for passive investing? I don't think so, as sometimes there will be some tokens/coins that have a lower price than the average index that could increased more than 300% in just several months. CMIIW.