dont think that there is any such thing as ready mixed mortar.... it sets chemically, not in contact with the air so would just go off in the tub. unless you are referring to the sand/cement mix that you can buy that needs water, in which case that should do the job fine. PVA the existing hearth first so that it bonds properly and make the mix fairly wet so that it levels easier.
a simpler alternative may be levelling compound. on such a small area it wouldnt be much hard work at all. bear in mind that either will take weeks to fully cure.
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