Are Christians welcome at Jewish funerals?

Yes if the purpose is to mourn the loss of a loved one or friend, or to comfort and support the mourners because of a connection to the deceased or to the mourners. It is very commonplace -- and it is not limited to Christians -- people of all faiths who have those purposes are welcome. Is there a family that might take exception? Of course, but that would be true for any faith based on personal choice. For instance, sometimes a family will have a family or private funeral only; then nobody other than the specific invitees are welcome.