I think he might just be hitting a rookie wall. His offense is a work in progress. It takes a while for some players. Just look at Avery Bradley. I think Smarts ceiling is higher than Bradley's. Bradley was a mess his rookie year and look at him now.
Coming to say the same thing about he rookie wall,but define it differently. NBA rookies have never played so much basketball in their life, not by a large margin. It's really common for rookies who've had major minutes all year long to fade down the stretch. This is a lot of nothing to be concerned about. Smart's mentally and physically exhausted. Stevens will try to get him to fight through it, but sometimes that means easing up a bit first.
Smart has played the 6th-most minutes of all rookies, and 5th-most per game. He's the only rookie in the top-10 of minutes per game who's in a playoff race, which is much more tiring than playing on an assured lottery team. We've probably seen the best a rookie Smart can offer, but long-term, there's little to worry about, unless you just like worrying.
EDIT: Bogdanovic is also top-10 in minutes on a playoff contender, but he's 25 and been a professional for years, so it's a little different.