That’s not likely a mutation (in the genetic sense), but rather a developmental anomaly.
Few evolutionary biologists would talk of mutations or variations being adaptive (or not). That term is better suited for the trait as a whole (i.e., the trait is adaptive – or not – and variations occur around the average phenotype; those variations are then selected for or against. As the frequency of the positive alleles for a trait increase in a population, that population is said to adapt).
True, but such things result in such rapid environmental changes that nothing can adapt to the intermediate environmental stages (e.g., the impact, the ensuing firestorms, etc.). You (as a population) are either pre-equipped to survive the disaster and weather out the subsequent chaotic period, or you aren’t. You don’t really get the chance to adapt in between, regardless of your genes.
And, of course, since you (as a population…) don’t really know that an asteroid is impending, you can’t really be faulted for not being pre-equipped to deal with such.
For many snakes, camouflage is not important – quite the opposite. If you’re a poisonous snake, you want to advertise that with bright colors, to keep predators away. It’s a common saying in the jungle that the more brightly colored a snake is, the more you should avoid it.
This also isn’t limited to the snakes. A skunk’s colors are anything but camouflage but predators mostly avoid the adults, looking for less messy prey.