First I just want to say that gills very a lot! They can be short they can be long they can be full day can be scarce. How long have you had this axolotl? Do you know how old this Axolotl is? Have the gills been more full? By full mean the filaments coming off the rami. Has there been a deterioration in the filaments coming off the rami? You're Axolotl could be in perfect health it just depends on the traits that it was born with. So as long as the gills of always look this way then I think the Axolotl looks perfectly healthy. However if the filaments seem to be deteriorating in your opinion, and there are fewer filaments on the Rami, then this would draw huge concern.