Hi axleandrose,
Floating in axolotls is one of those great axolotl mysteries. It can be completely normal or on the other end of the spectrum, be indicative of illness or stress. You would have to consider any other accompanying signs to make the call.
Now how often do you see it floating and for how long a duration? Does it sink back after a while? Does it have other signs of illness like inappetance, 'twitchy' and 'frantic' behaviour, unusual skin growth or discoloration?
In normal healthy axolotls, they sometimes float for a bit to get rid of excess air. Yes they do burp (eructate sounds classier) and also expel gases from the other non bitey end when they poop. They wound normally float down after expelling these gases.
Stressed axolotls can also float. They get stressed when water temperature is too high (ideal around 18 degree celsius), water chemistry is bad (You should have 0 ammonia, 0 nitrite, <60 nitrate, neutral pH), excessive water currents or nasty aggressive tank mates. You should try monitor and address those issues as well as provide hiding places for your axie to keep them stress free.
Ill axolotls can also float. If there is gill pathology, they may go to the surface to breathe more frequently as their sick gills isn't as efficient at gaseous exchange as before. Some types of parasites (such as anchorworm) can be extremely irritating to axolotls and cause them to swim to the surface more frequently. Axolotls undergoing the very rare spontaneous metamorphosis can also float in a bid to get to land. However in all of these cases, they would usually present with other accompanying signs like inappetance, unusual growth on skin etc.
So your special mission now is to try distinguish them. Observe your axolotl, take photos if in doubt. Im hoping your axie is just waiting to get rid of excess air.
Cheers