I think two things are making people think the Rifleman is a piece of crap, but in universe those two things would be much less of an issue.
1) At 60 tons, the Rifleman is a heavy mech. And so when you compare it to other heavies, it kinda sucks. I know if you gave me the choice between a Thunderbolt and a Rifleman, that's no choice at all. Take the T-bolt every time. But in universe, those distinctions (particularly in edge cases) wouldn't be as important. Compare it to mechs like the Trebuchet (the Rifleman has equal armor to the 5N, half a ton less than the 5J, and a lot more firepower). It has 1T less armor than the Centurion, but more firepower. If you think of it as a lightweight heavy, yeah it's thin on protection. If you think of it as an overweight medium, then it looks much better.
People in universe wouldn't have to allocate more character points to purchase a mech that was in a heavier weight class. The Rifleman being a bit overweight would be seen as a good thing. More internal structure. They wouldn't say "oh, if only it was a medium, then it would be good". I'd take a Rifleman over a Wyvern or a Hatchetman any day.
2) A lot of the early mech designs were fairly light on armor. Compare a Rifleman to later era mechs when FASA and Catalyst started giving almost everything max armor and yeah, it sucks. But those designs wouldn't have been around for most of the mech's lifespan. The Warhammer had 10 tons of standard armor. The Marauder had 11.5. And these are supposed to be badass combat machines that everybody feared. Armor was thinner, to-hit odds were worse, heat was out of control, ammo blew up. Those were common problems in 3025 play. The Rifleman does okay compared to the non-optimized designs of the day.
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I've had pretty good success using the regular old 3N alongside an in-your-face mech like the Hunchback, or a fast mover like the Phoenix Hawk. Park your Rifleman in a good spot (preferably woods with good LOS), and send the other mechs in close. Your opponent will normally shoot at the target that has better mods. So the Rifleman just sits there undisturbed and provides supporting fire. Don't be afraid to go up heat (sometimes a LOT of heat) if you get really good to-hit numbers one round.