So we know that in the Potterverse they celebrate Christmas.
No, I don't think that one can definitively conclude that at all. They celebrate Yuletide, but could be calling it Christmas out of habit of associating with Muggle-born witches and wizards. Father Christmas/Santa is never mentioned in the HP books. Also, Yule Ball is used in book 4, not Christmas Ball.
Hopefully most people on here know the original reason for Christmas is the supposed birth of Jesus.
Actually Yuletide, the pagan celebration of the winter solstice is far older, and also involves veneration of holly and yew (and use of mistletoe).
What is Jesus considered to be when taken from an in-universe point of view?
A study (I forget which journal) showed HP mentioned God, Lord, Merlin many times but never Jesus. Lord can't be taken in a necessarily religious sense because it can refer to a royal title, as in 'Lord Voldemort'. Jesus appears to have no role in the HP books, though a direct quote from one of his sermons is used in the 7th book. However the choice is problematic as the expression 'where your heart is, there you will also find your treasure' may not be originally Christian. It sounds like a pagan expression of the time that was picked up by Jesus. Pagan expressions often involve the idea of hidden treasure.
Is Jesus considered a wizard by the wizarding community or the author/narrator?
It is highly likely that the wizards consider all miracle workers/ prophets as magic users, though their magic may be untrained, and attributed by themselves to divine influence. The reference to Sybill (a prominent prophetess in Rome) seems to point to that interpretation. The use of Saint in St Mungo's is not conclusive evidence of Christianity in the books, as pagan sites were often shrines for the early church.