Banksy is (was?) straightforwardly good, it's just that he's not great, and he's not deep, and he kept riding the confusingly arbitrary rocket to art stardom far beyond the point where his actual work could support it, which makes him look like a self-important sellout. He wants to be the same guy he was originally while also having all this, and that guy just does not make sense with all this. But that guy was good, in his original context!
You have to look at it in the context of the 90s, right? In the 70s, UK and US culture fermented to the point where it was starting to get a bit ripe, and people voted in fusionist conservatives to cut it open, scoop out all the substance, and fill it with consumerism and family values. By the 90s, the consumerism was still intense, but the family values were on the way out, without other values to replace them.
The iconic art form of the 90s was the advertisement. Banksy was the kind of guy who could go into advertising, who was good at basically that sort of concise, pithy "message" image, but was resentful of the whole complex. As a result, he applied those skills to, like, guerrilla marketing left-wing social conscience in the same idiom. The 90s was the era of irony and ambivalence, and its youth social conscience habitually had this weird affinity with commodity trash, reflecting a sense that everything had already been captured and it was uncertain what it really meant to still care about things (other than brands) at the End of History.
Anyway. There's an actual craft to the kind of stuff he was doing, but it's still low art. It pricks the skin and catches the eye but it's never, ever deep, because it can only evoke simple ideas that people already hold. This worked well for his original schtick, when he was an unknown street artist mostly working in a single town, but trying to parlay that into a reputation and career while doing the same schtick just turns you into an editorial cartoonst. The appeal was surprise and authenticity, and how can you possibly maintain that when you have a reputation and a brand identity and a swarm of wealthy collectors and international newsmedia reporting on your every move? He had to either reject all that or else pivot to something that complemented it, and he couldn't choose between the values and the money, and so...uhm. Actually it worked out great? He's one of the world's most famous and sought-after artists, and he presumably feels OK about his values, so. You know! But yes, it makes him kind of a parody of what he originally was, now.
In a sense, I feel like this itself is artistic, because the culture he emerged from was also very concerned with Selling Out, and so being morally and aesthetically undermined by his own success creates a sense of aesthetic completion. But personally, I don't like that he's still bucking against it in a half-assed way. Propriety demands that at this point he either burn it all down catastrophically in a crazy way, or do an ad for Coke without trying to be cute about it and salvage his dignity somehow.
