I'd change the above to suggest it's not interesting for the plot of a
scifi novel. And I'm not even sure I agree with myself about
that either even just as I've written it.
The first point being that there's an infinite number of interesting stories you could tell about ordinary people living ordinary (to them) Culture lives doing all kinds of exciting things. There'll still be conflict - just different kinds. E.g. people will still fall in love with the same person, or grow to hate someone they once loved over petty little things, and a myriad other things that would produce conflict. Just not about material wealth etc. But many of these stories would only notionally be scifi in terms of setting.
Writing a scifi novel set in the core of the Culture would be harder than setting it on the edges. You'd lose a whole lot of sub-genres. No cyberpunk or military scifi for example. But you could still find stories where the scifi setting matters.
Part of what makes the 'edges' of these utopias easier to write about, though, is that you get to write about them as someone peering in. It justifies more world-building and explanations that helps relate the stories to our reality. A lot of stories about utopias are in a sense travellers tales - all the way back to Thomas More's original book Utopia, for which the concept is named.
And as with More's Utopia, it's easier for us to relate when there are ambiguities and cracks. Not least because pretending there are none is difficult given people have different ideas about what is ideal.
With respect to Banks, for example, I regularly come across people in discussions who have trouble dealing with the fact that Banks was a very outspoken socialist, at one point endorsing the Scottish Socialist Party, and who find it hard to come to terms with a socialist describing a society such as the Culture as an ideal to aspire to. To some people that in itself somehow mars the Culture for them.
And incidentally a lot of the time when people try to imply the Culture is not an utopia, they try to find authoritarianism lurking in the shadows, but forget that for every imagined slave-driver, for example, the Culture canon describes whole sub-civilizations of billions of people freely and peacefully not just dissenting but deciding to leave the Culture over political disagreements without any attempt to use force to hold them back. Any perceived authoritarianism in the Culture exists only in so far as its citizens willingly continues to choose to subject themselves to it.
And that last point brings it back around to the question you asked:
If it's not interesting and enjoyable to live that way, nothing in the Culture prevents you from packing your metaphorical bag and leaving, be it alone, with a group of friends, or seceding with a whole Orbital to forge another path.
That, to me is the strongest evidence that whatever musings we might make, in universe the Culture must be a pretty decent place to live, or it'd disintegrate with people forming their own little fiefdoms. So we might well speculate about whether or not we would like to live in it. But Culture canon is that trillions of citizens choose to stay despite having all the material wealth and practical opportunity to leave if they want to.
If it's a cage, it's extremely gilded.
At the same time, we know people can leave because we also know the Culture is not perfect for everyone, because some people have left the Culture fully or partially. But the very existence of a continuum - the Culture Ulterior consisting of factions that are mostly separate but still nominally Culture - also helps drive home that we're not talking about a centrally ruled empire, but a sprawling decentralised culture, and there's an infinite variety of levels of "leaving" too.