Laurens said:2 Peter 1:16 states:
For we did not follow cleverly devised myths when we made known to you the power and coming of our Lord Jesus Christ, but we had been eyewitnesses of his majesty
This seems like a reference to critics saying that Christianity is a 'cleverly devised myth'. Whether these were Jews or not is anyone's guess, but it seems that someone was making this argument.
This seems like a rather ambiguous quote and I'm not sure whether you're arguing that it supports a mythicist position, or that it just evidences the existence of counter Christian arguments at the time of writing. Yes, it could be in reference to what Jews might have been saying about Christianity as you suggest it could. However, there are numerous potential possibilities, all seemingly reasonable:
-Any group, person or people could have been making arguments about myths, who perhaps this line was referring to. It could equally have been that it was referring to nobody, but is simply an idiom, or written for emphasis.
-Myths or stories can take various forms. They are not necessarily explicitly or completely fictional. So they can be entirely fabricated, or they can be based in a decent portion of fact, or anywhere in between. If anyone was making arguments about myths, it is unknown to what degree these arguments were made. For example, perhaps the detractors believed that Jesus had been concocted from thin air, or perhaps they referred to stories about his life being mythological. It is unsurprising that arguments against Christianity would have existed at the time, given the mixture of religious beliefs around. What's unclear from this reference is exactly how far any such arguments would have gone.
-It is unknown from this reference whether any such arguments of mythmaking would have been directed at Christianity specifically. Perhaps he had heard people speaking sceptically of other positions, religions, groups, etc. and wanted to affirm that Christianity was no such position.
In short, there are several variables in play here. Yes, the quote from Peter could reference extant positions of Jesus mythicism, but much more likely is that it’s in reference to one of the other possibilities. If it was in reference to detractors who were accusing Christianity specifically of being fictional, it is not specific about what degree and it does not specifically reference arguments being made about the non-existence of Jesus. Perhaps we can’t rule this out, but at best we can only rule in that it references counter arguments to Christianity, which might have taken any form.