Smart is a better player than dieng so no. Dieng is average at everything, Smart is utterly elite at one thing, and hasn't lived up to his potential at the other.
I think "Utterly elite" is a bit of an overstatement.
He's a very good defensive player who has the potential to one day be an elite one. He's not an elite one just yet - I think he is a little bit limited sometimes by his lack of raw lateral quickness, outright athleticism, and (to a degree) conditioning. It's because of this that I feel he occasionally struggles to stick with the quickest of guards.
He's a very intelligent and very versatile defensive player, and he seems to make up for his lack of elite quickness by being very good at predicting where a guy is going to go, and beating him there. Every now and then however, when he does get beaten, sometimes he seems to lack toe first-step quickness to catch up.
This is in stark contrast to Avery Bradley, who's incredible quickness, conditioning and motor is what allows him to play extreme pressure defence on even the quickest guards - however I don't think he's got Smart's defensive IQ off the ball, or Smarts' positional versatility.
To be fair, I think Bradley's defensive contribution is harder to replace then Smarts. There are other guards and wings out there who can defend 1-3 (e.g. Butler, Winslow, Crowder) but there are very few (if any) quick guards who can apply full court pressure on lightly quick point guards the way Bradley can. That's a really rare talent.
I'm not as high on Dieng as others - but we need a big man pretty desperately. I'd consider trading smart for him straight up (especially now that we have Jackson / Rozier) but if you're throwing anything else in there you're really overpaying.