In fairness to Dirk, if his Mavs team was in the east for most of Pierce's career, there would have been multiple final appearances.
I really don't think you can draw that conclusion at all. The Mavs have failed against several lesser teams through the playoffs the last decade and just because they were in a more competative conference does not necessarily mean that they would have "multiple final appearances". They do have multiple appearances in the finals right now in the West but you have no idea what it would have been like in the East.
Dallas, V Eastern teams 2010: 22-8 (.730, top 3 in NBA against Eastern teams)
Dallas, V Eastern teams 2009: 21-9 (.700, top 5 in NBA against Eastern teams)
Dallas, V Eastern teams 2008: 18-12 (.600, 12th)
Dallas, V Eastern teams 2007: 27-3 (.900, 1st in NBA against Eastern teams)
Dallas, V Eastern teams 2006: 27-3 (.900, 1st in NBA against Eastern teams)
Dallas, V Eastern teams 2005: 27-3 (.900, 3rd in NBA against Eastern teams)
That's pretty compelling evidence for Dallas doing better in the East.