That would work if it was bringing something to the table that a lighter mech couldn't. However, the answer to that question is no: a lighter mech can carry the same or better firepower at the same speed and meet or beat the Man O' War's armor level. Therefore, the Man O' War Prime especially is objectively a bad mech. This doesn't make it a useless mech, it still can do the job, but the question being asked was whether it was a good mech, not whether it was a useful mech.
I'm going to disagree with you a bit here. The Man O' War isn't a perfect design. It's an 80 tonner, after all. And there's basically nothing an 80 tonner can do that can't be made more efficient by going either up or down 5 more tons. So it isn't perfect, but that's a standard that very few mechs can ever achieve. A mech can be "good" without being 100% optimal.
It seems to me that the Man O' War Prime would have been a response to Inner Sphere vehicle units. Especially under the old vehicle damage rules, two LB-5 Xs would be murderous. The Man O' War's job would have been to engage vehicles outside of their range and fire until its ammo bins ran dry. It could engage with Warrior VTOLs as well, since the LB-5 has the same range as the AC-2.
From that standpoint, it's pretty good. It's not the absolute best -- it'll never be a MadCat, that mech is just a better chassis. The MadCat hits all the design sweet spots and the 80 tonner doesn't. But you really don't want to equip your MadCats for the limited role that the Man O' War Prime is fulfilling. It is serving a necessary purpose in warfare, but not a glorious one. But it's still necessary, and it's very good at that job.
What it boils down to is the Man O' War is a solid design in most of its configurations. It's just not quite a MadCat. So when you invade the Inner Sphere and find they've got hordes of vehicles, you need a heavy cavalry mech that can tear apart a tank company and swat away VTOLs. So you take your "almost a MadCat" mech and give it a very specialized loadout. And then it ends up being so useful that the Inner Sphere commanders think of that as its prime configuration, not knowing that it was basically an ad-hoc solution to the vehicle problem.
So is it good? Yes, it's very good at what it's supposed to do. But it's not the most efficient mech possible, because of in-universe reasons.