No, doing UBI via cryptocurrency is just a bad idea. To start with, UBI fundamentally needs to be centralized because a decentralized system can't determine who the unique humans are. You need to either rely on existing centralized government databases (which may also be flawed/corrupted...) or you need your own centralized testing personnel/devices.

Second, a decentralized cryptocurrency can only make money for "the system" through very blunt/stupid methods such as inflation and transaction fees, and this doesn't scale much. People won't use a cryptocurrency that's going to inflate a lot: that'd be stupid, and people aren't that stupid. That's why you don't usually see governments try to fund themselves entirely through inflation, and when they do, it quickly leads to hyperinflation from people increasingly+exponentially refusing to use that unreliable money. If you had a cryptocurrency that had significant stand-alone merit prompting people to use it (eg. BTC, ETH, XMR when they were first created, not new clones), then maybe you could get away with taking a 0.1-1.0% inflation tax and doing something with it, but you're not going to achieve UBI with that. UBI requires some other form of taxation, probably one requiring violence via a state. (Voluntary taxation communities almost always fail, look at various attempted communes such as eg. Kibbutzim in Israel.)

Looking at Manna as an example, their whole UBI aspect relies on a centralized nonprofit; what then is even the point of the cryptocurrency aspect? What value does it add on top of just setting up some BTC-accepting charity that would send donated funds to all verified individuals equally?

IMO a better idea would be to compete with Patreon: subsidize non-traditional work so that people can escape the 9-to-5 nonsense. But even then, I suspect that creating a new cryptocurrency would be counter-productive; you don't need to shoe-horn a new coin into every little thing.