in opensea for example, i see some art thieves not only one or two but very often
Such things are inevitable in such a so-called decentralized network. We will keep seeing how those kind of scam and theft are keep popping up just like how many scams/shitcoins are still flourishing to this day. The only way that Opensea is able to get over it is just simply delisting the NFT from their end.
~, and terrible art but gain more popularity and high value.
I can't say anything but it comes down to the buyer perspective, the owner might find the aesthetics of the art isn't subjectively relied only on how it looks, as the interview on the video has already been addressed. But some also can say that the art itself is an utter shits, that the value does not represent the aesthetics. It really comes to as subjectivity. As a matter of the famous saying, "In matters of taste, there can be no disputes".