The Makuta being in Karda nui did not impact their appearances. The four Makuta which made up the Phantoka wave of sets all chose to look like that deliberately - they could choose to look different at any point. Antroz entering the Jetrax T6 altered his appearance automatically to a pilot form, in a somewhat similar manner to the Toa Nuva’s adaptive armor.
The Makuta in the Mistika wave, however, were mutated by the waters of the swamp of secrets, although it isn’t clear how much of their appearances was a deliberate choice and how much they were mutated, but it did lock all three of the in their respective forms. No matter where they theoretically might go, they would be unable to alter it simply by changing location.