I'm not sure this solves any problem users have. Due to the nature of Signal requiring the phone number of the counter part to be in the phone book this feature will mostly be usable with friends or people you know directly. A feature which some users might want is to easily send money to a friend. These transfers will therefor be inherently be based on the countries currency. I'm not sure people will accept a cryptocurrency which value fluctuates in relation to the normal currency.
For other uses cases they can use a cryptocurrency directly (even MobileCoin) from another app. I can understand that cryptocurrencies are very interesting from a technical and cryptographic point of view. But in their current state most of them a more for speculation and doing state of the art computer science.
Therefore I think this totally useless in the signal app.