No, Lookout and any app / mobile security providers cannot protect against Stagefright. Lookout can protect your device from mobile malware delivered that could potentially leverage Stagefright exploits. Keep in mind that a device will remain vulnerable until it receives a patch for these vulnerabilities.
Android devices other than Nexus devices will ultimately need to get these patches through a Google partner (either a device manufacturer or wireless carrier). Nexus devices, however, will receive a direct security update from Google next week, according to a Google spokesperson. Unfortunately, security patches delivered by Google’s partners can take weeks and even months to fully deploy, and are typically delivered through a System Update.
When these patches become available, you should download and install them immediately. To check if a patch is available for most Android devices, go to Settings and click System Updates