Well, I am by no means an expert on the fluffiness of gills, However, we could look at it from a logical standpoint and make a couple assumptions.
There are generally two factors when assessing traits. Genetic factors and environmental factors. While I am certain that you could potentially breed for fluffy gills, it would be difficult to look at an individuals ancestry without accounting for environmental factors. So we can dismiss this one for now and look at what we can control.
Environment.
We can start by acknowledging that gills are a very sensitive organ. They are easy to damage so if you want to maximize gill fluff, minimizing the things that damage them is key.
Chemical damage: Ammonia, nitrite, nitrates, and other dissolved waste generated materials in the water collumn can and will damage gill tissue and slow regenerative growth. So the water should be kept as pristine as possible with good water changes. It should also be noted that we only test for a small number of parameters with the water, heavy metals, and other dissolved organics play a roll.
Cohabitants: fluffy gills make a tempting looking snack to anything that enjoys moving prey (which is just about every aquatic species I can think of). If you want long and fluffy gills, keep the axolotl separate from other species.
So in general, I'd say the keep to fluffy gills is water quality and nothing nipping them.
As for conditioners, just don't use them. Conditioners generally have coating agents in them. I can't see coating the gills in something as being particularly good for them. A healthy animal shouldn't need anything in the conditioner anyways. (unless it is dechlore of course).