Bored at times and plays less than stellar defense? People said the same thing about Dirk/Pierce/Carmelo.
Jabari just turned 19, he can learn to defend.
There's a difference between being described as someone who has the tools to play defense but lacks effort and being described as someone who might lack the quickness to guard SFs and the strength/size to guard PFs.
Part of the concern about Parker's defense is that he fits the classic tweener forward description where it's not clear what position he should be guarding in the NBA. It's possible that he can produce incredible offense that outweighs defensive liabilities. It's possible that losing some weight will improve his lateral quickness so that he can guard SFs on the perimeter or gaining some muscle will make him able to hold his own against bigger PFs.
Take a look at the offensively gifted combo fowards drafted in the top five who lacked a clear position and try to figure out if Parker looks more like the ones who succeeded or the ones who failed.
How does Parker lack a clear position? Right now, pending improved conditioning, he's a defensively challenged SF with advanced offensive ability. That doesn't make him a tweener.
What players are you referencing? Let's see. Derrick Williams is an obvious one and nothing like Parker because Williams is both a tweener defensively AND offensively. Williams is largely a catch and shoot player who isn't all that good at catching and shooting. Anthony Bennett is another one and again nothing like Parker because he's a tweener both offensively and defensively. I'm sensing a trend here. Similar to Williams, Bennett lacks the handle, quickness and diverse shot making to hack it at small forward and he's challenged to guard either small forwards or power forwards. He was a puzzling choice by the fired Cavs GM.
Beasley is another reference point but he hasn't busted because he's not skilled enough or can't defend (defense hasn't helped of course) but because he's been a black hole who simply hasn't been a good enough shot maker. It's not for a lack of skill. He can handle, shoot from distance, create in a variety of ways but he shies away from contact and loves jump shots most of all. If Parker goes the way of Beasley, it's not going to be because he was a "tweener."
See, the thing is, I'm not seeing where your tweener label comes from. Is Kevin Love a tweener? I mean, he can't guard SFs or PFs either. Being bad on defense is a negative to be sure but Jabari has the prototypical skill level of a wing. If a team has a player who can go coast to coast off the dribble, handle in pick and roll and isolation situations, post up, shoot from distance and the mid-range and he stands 6'8ish, they'll slot him in at SF hopefully next to a defensive wing who can take tougher matchups. If he plays some PF, it'll be because he might have an even greater offensive advantage and not because he has to like with Derrick Williams or Anthony Bennett.