Lamar is a way better pocket passer than his doubters give him credit for. He'll stay in the pocket and look for receivers way longer than most "dual threat" guys. He just hasn't been good in the playoffs thus far. Better touch passer than any of the other guys that run a lot too.
People also need to look at his cast. He hasn't had the luxury of an elite, in their prime receiver. The only constant for him has been Andrews when he's healthy. He's also played in an offense that is notoriously balanced and run-centric by today's standards. One would be naive to say Jackson himself hasn't factored into this (as he has), but they are still run heavy. When you have leads, a good to great offensive line (routinely), and a lack of WR threats that's probably the right move.
Take a look here. Baltimore has been in the bottom 3 in run/pass percentage, routinely the last several years. If you go back to when Flacco was there they usually sat close to 20th save for a few aberrational seasons. When the team is good, and they are routinely in the lead, Harbaugh likes to run more than pass. Simply put, Lamar's stats aren't all "on Lamar".
NFL Football Stats - NFL Team Passing Play Percentage | TeamRankings.com
NFL Football passing play percentage, by team.