Generally, cryptocurrency portfolio diversification reduces the risk But comes out with a lower return compared to the opposite strategy.
Diversifying is a good idea, but doing it in unsystematic way is such a YOLO idea. That's when it becomes very risky.
This is a principle that works in the stock markets, but works opposite in crypto. Bitcoin and Ethereum are the safest of the assets, and the further down you go in market cap the more risk you present. Nearly all alts move with bitcoin still, just with exaggerated upsides and downsides. To say that diversification is safer would imply that some altcoins move differently, but they all really move the same, generally together. There are some times when alts move instead of bitcoin, but in general if you just stuck with BTC or ETH and avoided all other alts you would be much more likely to have consistent gains.