A 'Mech has 11 independent armor and structure locations so using lots of weak weapons will tend to just gradually grind them down. The best way to deal with a big 'Mech is to knock it over with stability, or to focus on one side with as much punch as you can get. Side shots are very good at removing arms, side torsos, and legs, but there’s no point in going for shots from two different sides.
The emphasis on damaging the structure under the armor is also what makes the big single-hit guns like the PPC and AC/20 worth all their tradeoffs. An LRM20 can do 80 points of damage, but that damage will be randomly distributed over as many as 8 different locations. An AC/20 hits for 100 in one spot, which means it’s EXTREMLY likely to punch through and hit structure, and hitting structure gives you a hope for a critical or, even better, an ammo explosion.
Incidentally this is part of why I don’t really care for the Battlemaster or the Hunchback 4P - a ton of medium lasers looks cool but is bad at decapitation.
but in general, I find that stability damage is extremely good. Kill the meat, save the metal.