Curious what his defensive ceiling is. DPOY depends a lot on team success and optics - and typically, the winner is the big man who anchors an elite team defense. The exceptions since 2000 have been Kawhi (twice) and Metta World Peace. Last guard who won DPOY was Gary Payton in 1995-96. You can see the list since 2000 below (basketball-reference.com)
Smart received fourth-most points for All-Defense and 4 3rd-place DPOY votes. 
I think the (narrow) path for Smart as DPOY is:
- Celtics top-5 team defense
- Smart as starting PG
- Reputation for hustle, harassment, switchability, and general terror overshadows reputation as flopper
- Celtics are a clear playoff team
2018-19   Rudy Gobert      
2017-18   Rudy Gobert   
2016-17   Draymond Green   
2015-16   Kawhi Leonard
2014-15   Kawhi Leonard   
2013-14   Joakim Noah   
2012-13   Marc Gasol   
2011-12   Tyson Chandler   
2010-11   Dwight Howard   
2009-10   Dwight Howard   
2008-09   Dwight Howard   
2007-08   Kevin Garnett   
2006-07   Marcus Camby   
2005-06   Ben Wallace   
2004-05   Ben Wallace
2003-04   Metta World Peace
2002-03   Ben Wallace
2001-02   Ben Wallace
2000-01   Dikembe Mutombo
1999-00   Alonzo Mourning