Is there any problem with using a construct of a giant octopus as a mount? I know the anatomy isn’t perfect but I’m thinking with it being made by the artificer, that could be part of the design. It would still be super slow on land. Is it just DM dependent?
The Giant Octopus can absolutely be used as the Artificer's Mechanical Servant, as it is a Large beast with a CR less than 2. However, nothing in the Mechanical Servant ability (or the construct type it grants) will remove this trait from it:
Water Breathing. The octopus can breathe only underwater.
This will undoubtedly cause you some problems, even with its corresponding ability to hold its breath for an hour. Even if you overcome this, the rules for riding a mount merely say that the creature must have "an appropriate anatomy". As you guessed, this leaves it as the sole purview of the DM to determine whether you can ride your Giant Octopus.