I believe mempool.space ranks miner health based on how similar their blocks are to the ones predicted by mempool.space's tx selection algorithm, which is itself based on the one in Bitcoin Core. Many Ocean miners run Knots, which has a different selection algorithm by default (it ignores certain types of spam transactions), so lots of Knots blocks don't match mempool.space's expectations, and I think that's why they give them a relatively low health score