Such a function comes against considerable problems (ie it is falsified) with distant objects of known distance and duration (like the spin of a pulsar or the decay of isotopes), since we do not see the time for those events lengthening as they should do were the light travelling from them to have slowed down on its journey. Instead the perceived time for those events lengthens by the very much smaller amount predicted by the red-shift of the expanding 14 billion yo universe and mainstream einsteinian theory. Also you cannot just alter c without having all kinds of other effects. c does not exist in isolation. We cannot see any such effects when we look into the past.
Here is a fuller exposition of the problems for the argument that light slowed down on its journey if you are interested. Basically the premise doesn't hold up against critical examination. The only predictions it makes are falsified.
I think it is premature to write it off. Cosmologists use concepts such as dark matter and dark energy to explain the universe. These concepts are introduced because the known theories are incomplete. Perhaps with further investigation Creationists will be able to explain it. Even if the Creationists are wrong, it forces scientists to think outside the box enabling science to develop -- which is a good thing.