From Calvin's Institutes, Book II, chapter 17
"Therefore when we treat of the merit of Christ, we do not place the beginning in him, but we ascend to the ordination of God as the primary cause, because of his mere good pleasure he appointed a Mediator to purchase salvation for us.
"...The free favour of God is as fitly opposed to our works as is the obedience of Christ, both in their order: for Christ could not merit anything save by the good pleasure of God..."
So... we can speak of the merit of Christ, but only if we understand it as not totally-on-His-own obedience apart from the Father.
Merit as commonly understood means deserving and rules out gratitude. Christ actively and completely obeyed His Father, and that obedience is imputed to us. But to call it merit implies an all-on-His-own-ness that isn't true. Father and Son are two parties in a covenant to save us, and one in essence, but they are not two parties to a contract, held together only by the terms of that covenant.