(On a related subject, in mech packs, the special +

A Pay 2 Lose hero is, in my view, not simply a hero variant of a bad chassis. It's a bad variant of any chassis.
But what really inspired me to make this thread is a twofer: A bad hero of a bad chassis, the St. Ives' Blues. It's like any other Vindicator -- light armor and a low engine cap -- only with fewer hardpoints. Oh, it gets a minor engine bump, but not nearly as much as the 1AA. Its main benefit is that with a 250, you don't need external heat sinks. Then again, the standard heat sinks, standard structure and standard armor it comes with leave plenty of room.
The Ember. While the FS9-S, K and A were running around like Inner Sphere Arctic Cheetahs before they were cool, boating small or medium lasers and vaporizing Atlas rear armor, the Ember was plinking away with its machine guns.
I know some people like to romanticize ammo explosions from MGs, but while you were staring deeply into your opponent's foot actuators, someone with real weapons destroyed his CT outright (or perhaps yours) in less time and with less risk. And that person was actually useful for the entire match, not just when there were exposed components containing ammo. Fun fact: Machine guns don't do any extra damage to internal structure. Critical hits do damage to equipment only.
You are better off with unused tonnage than you are by filling it with machine guns. They are more likely to lead you to a fatal distraction than to victory against anyone.
For a long time the Ember didn't even have machine gun quirks, just languishing with -10% laser duration while Firestarters with more lasers got better quirks for them. Now that it has a +25% machine gun rate-of-fire quirk, it has also been given +15% flamer range, which really seems like a troll.
The Huggin. Repeat everything I just said about machine guns, then add that this thing doesn't even have energy hard points. Not that you need them, exactly, but light mechs generally depend on them for firepower, and this one doesn't have much to make up for their absence. And it has two 20-tube missile launchers, for some reason. Like PGI wants you to make an LRM40 Raven. Or maybe some kind of LRM20 + SRM6 abomination.
And the RVN-3L has ECM, obviously. That seems like a cheap shot to make against the Huggin, but I'll make it anyway. We could make the same criticism of the CDA-X5 because the 3M has ECM, but the X-5 isn't bad, it's just weird that it is the only mech in the game without AMS. The Anasi doesn't have the SDR-5D's ECM, either, and it only escapes this list because the 5K and 5V are even worse.
I believe there are other contenders, such as the Pretty Baby, but I am less familiar with them. I think an honorable mention goes to the IV-Four for focusing on low-slung ballistics, the weight of which also prevents you from taking advantage of the Quickdraw's speed, so you may as well be piloting one of a gazillion other heavies.