Well, there is the religious barrier between Catholics and Orthodoxes to consider and the fact that there will likely be more imminent problems for Hungary-Naples, both in Italy and possibly from Bohemia and Poland.
In any case the battle was very close iotl and, while the fact that the Ottomans have great reserves even if that army is destroyed still stands, a possible Serbian victory is bound to cause interesting divergences, for example in relation to a
Nikopolis equivalent.
Also isn't there a large minority of Orthodox Christians in the very south of Italy e.g. Apulia and Calabria?