That’s a quite interesting issue. Of course, we could think round based coinjoins are benefiting from Schnorr a lot more than JoinMarket transactions, because there you aggregate a lot more signatures. And of course it is true, but Schnorr will be an implementation nightmare, because it is interactive.
For example, right now, in Wasabi, at Signing phase, the coordinator gives the transaction to every peer and the peers submit back signatures. Finally the coordinator combines those signatures. But, with Schnorr it must go like this: the coordinator gives the transaction to the first peer, the first peer then sends back the signature. Then the coordinator gives the transaction and the signature to the second peer, the second peer signs with aggregation and gives back the signature. Then the coordinator gives the transaction and the already aggregated signature to the third peer, and so on… And then we have to handle timeout and other issues. Anyway, it’s probably workable, but it is an interactive protocol :(