If the mossad was able to plant explosives without being caught, I wouldn't be surprised if they also planted bugs (indiscriminately) in many electronic devices delivered to Lebanon such as TVs, computers, phones etc... Similar to the spy chips implants within the Supermicro server motherboards.
How much of it is an excellent question. It's remarkable that apparently (?) none of the devices went off prematurely and tipped off the targets. That implies a higher degree of QA than you'd expect from a more ramshackle organization.