Yes, the Lisbon Treaty will improve the EU's foreign policy, because it will put some credibility behind what the EU says. The Lisbon Treaty is meant to make the EU stronger as a governing body. Right now, the EU can more or less only make advisory opinions on foreign policies. This is better, because the EU will be able to have back up what it says.
The Lisbon Treaty is a move toward European Unity, by which the EU centralizes more power from the individual nations. This will strengthen the EU's foreign policy and improve it's relevance. Rather than being a simple free trade area, as originally intended, the EU is becoming more of a united government, consolidated government. This may hurt the foreign policy of individual EU member states, but it will improve the EU's foreign policy.