First of all, I will say the understand the importance of GW2 having this kind of multiplayer mount. There are people who will never touch this, I am one of them, but likewise this is a unique thing and it deserves to exists. That the only new mount this expansion has such niche appeal is extremely lame and kind of laughable. It's like expecting fishing to be a tentpole of your expansion.
Now for my feedback on what I could actually test which is the driver portion only... it's fine. It's too big and it gets caught on level geometry a lot. It handles like a truck which is unique but not particulary fun to me. Though if people are racing these things there must be some appeal to people. The flight is super useful for covering for the weaknesses of the mount getting caught on things but I would use just about any other mount most of the time.
Some more niche criticisms, I think it might be good to have one slam that works as it currently is and one slam that actually dismounts you so the character can start combat. The skin is very aesthetic but a little weird outside of the Canthan expansion it would be nice to have an easily earnable alt-skin that uses core-game aesthetics. Maybe hide it in a little quest even though I'm sure you'd just sell it.
In the end I don't see myself using the Seige Turtle very much but it is fundamently fine on a game design level.