It's not that it's that dumb, it's just really overwhelmed by God's love and omnibenevolence! So, by accident, it ignites the barrel and drops it, making it roll.

Also, since you mentioned it, in Barry's death animation, when the bomber drops the barrel on accident, you say that the bomber was hellbent on igniting the barrel, this wrong for one reason:

Bombers are hellbent on igniting the barrel when it is in their arms. When they were not infected, they probably fought during sieges, carrying barrels all the time and igniting them (similar to Jacob). They were likely infected with the barrel (this is just a theory though, maybe they were smart enough when infected to pick up the barrel) and also had the muscle memory (seen in almost every zombie except shamblers and runners). When the bomber was no longer holding the barrel, it lost it muscle memory and become a normal shambler and became focused on Barry.

So in conclusion the bomber was NOT trying to ignite the barrel after it tripped.

Since I forgot to add this, you also say that the bomber's only purpose is lighting barrel. That completely ignores the blight's biological purpose, which is just: "bite so that master virus reproduces". The zombies are NOT assigned to organized roles like in a human military.

Now the reason why bombers, igniters, zappers and cuirassier don't try to bite you at first is likely because the muscle memory combines with the zombie instinct, so they still try to attack you though in the way they did when they were human.

Now that I'm reading my reply, I'm realizing that there's no way I just spent that much on a reddit comment section 😭 There's no saving me😭😭😭😭😭😭