Ideally speaking, yes, Dalit Christians need not be given reservation, as Chrisitanity professes to be an egalitarian religion, whereas Indian origin religions like Hinduism, Buddhism and Sikhism, allegedly support casteism.
But the ground realities do not support this perception. As a person who is well familiar with members of the Christian communities of various denominations, I have seen that casteism is very much prevalent and alive in the followers of Christianity, especially in socialising, matrimony and employment in organisations owned by Christians.