The truth is, offense is much more valuable than defense. So while his lousy offense doesn't make his defense irrelevant, it is a much more important aspect of the game. In other words, great defense and lousy offense don't make an average player. It makes a below average player. On the other hand, there are many examples of great offensive players with bad defense that are still good players overall.
what defines "good" in your point of view?
because, in my opinion, "great defense and lousy offense" makes a good player.
and "great offense and bad defense" also makes a good player.
so, there is more than 1 way to be a good player.
Very good point. His opinion is just his perception.
I think most people including me agree with your opinion more.
I mean, he was perhaps the worst offensive player in the entire league last year. That's a pretty impressive feat. He's gotten a little better this year but he's still bad. Yet, people question how someone could consider him below average?
My point is pretty simple. Being a good defensive player doesn't cancel his offense. While sure, perhaps if others on the team make up for his offense can allow him to focus on what he does well. But 10 Marcus Smarts would be really really bad team.
10 ITs would be a really, really bad team that couldn't defend a team of 10 Marcus Smarts. I guess IT is even more below average than Smart. 
I guess you don't understand the analogy. Ten Marcus smarts at their positions (i.e., ten players with good defense and bad offense) would not be very bad. Ten Isaiah's Thomas's would be a [dang] could team because while there defense would be limited, they score a lot of points.
It doesn't matter how many points you score if you give up just as many points. Sure, I would say that offense is very marginally more important than defense, but it's not as drastic as you say. It's something like 51/49.
IT is a good opposite of Smart, because IT's offense is as elite as Smart's defense and IT's defense is as subpar as Smart's offense. Even with offense being marginally more important, I don't think you can claim Smart as being a below average player due to his subpar offense without also claiming IT as a below average player due to his subpar defense. That's just an inaccurate analogy and way to determine a player's status.
EDIT: Oh, and by the way, the saying "defense wins championships" is there for a reason. Statistically, at least since around the 2000's, winning championships has been more associated with defensive rating than offensive rating. Finals contestants have more often had a top-10 defense than a top-10 offense.
https://www.sportingcharts.com/articles/nba/offense-and-defensive-efficiency-of-nba-champions.aspx