What did you expect really? A 19 year old kid had an outstanding rookie season and spent an entire offseason hearing about how he was the second coming of every hall-of-famer since the inception of the NBA.
Add to that the "veteran presence" the Celtics brought in to guide Tatum is Marcus Morris. Now Tatum plays a lot like Morris - relying on a lot of ball stopping iso moves, chucking up bad 20 footers, etc - and everyone is shocked.
And let's not forget the weird "good son/bad son" routine Brad Stevens has played all year with Tatum vs Jaylen Brown where Brown could do no right and Tatum could do no wrong... I'd hardly call the consequences each faced for errors equitable. What is Tatum supposed to think he should be doing differently if he's still starting over Hayward (a max player) and Brown (another 3rd overall pick).
The kid's not any worse than he was a year ago, he just got a rude awakening to the world of the NBA. Everyone else is good too and if you're not improving they're going to move on without you.
It's more than likely he'll respond to the challenge this offseason - and this down season might have a silver lining in that he's most likely dropped his trade value enough that an Anthony Davis for Tatum plus pieces deal is now sort of laughable.