I think their defense is fine; they just got outplayed by the Celtics. When they beat the Wolves to qualify for the finals, that was already proof that they have a working defense, so let's hope they improve their weaknesses so we can see them in the finals again. Having Klay Thompson is a huge lift for their offense as they now have a complete threat both inside and outside. Klay is a shooter, consistent enough to bring some energy to the team, so that’s what I want to see from him in his first season with the Mavericks.
Klay Thompson is also an elite perimeter defender and I guess they are fine with their current roster and currently they became more better since aside with defense we also know that Klay is also well known for his shooting. Provably that his morale is so high since he need to prove something and Dallas is lucky to get him this season. Although many say that his performance decline last season but provably he can cope up with that and maybe he's just unmotivated to continue since he don't like to stay on GSW anymore.
Dallas is became interesting team next season and for sure lots of people will provably follow on how the new Dallas big three will perform good and if they would click together.