Isnt a problem if China has 68% of the hashrate?
China doesn't have 68% of hashrate, but 68% of hashrate comes from chinese pools. That's different.
and yes, it is certainly a problem.. this could be more decentralized, but that's not that bad.
Couldnt China do a 51% percent attack?
No. Chinese government does not control the pools. Pools are not that coordinated and if there war a slightly chance that this could happen, miners would just move to another pools.
Take a look at this graph from blockchain.com
source:https://www.blockchain.com/charts/pools
There are many different individuals who controls those pools. They are not the same people, and they do not own the hash power: miners can move to other pools at any time.