While the concept is neat, practically any crypto could be used as a universal income payment. Gridcoin is definitely not the most suited for the job.
Further, the difficulty is in using cryptocurrency to actually pay your living costs. Just like paying a German resident Thai Baht as a universal income, paying them Gridcoin also does not allow them to buy local goods and services.
Hopefully in future, the gap between cryptocurrency and the 'real world' will be bridged to allow anyone to spend cryptocurrency at their local supermarket, but until then this may be a pipe dream.
While it's true that you can't spend GRC in your local store, the coins still have monetary value.
What's in your opinion makes a crypto suitable for UBI?
A combination of:
No crypto really ticks the second box (yet!). However, there are many pegged assets on the BTS DEX that could very easily fit the first category.