The Inner Sphere has been at war for hundreds of years following the fall of the Star League.
Mechs, king of the battlefield have been restored, rebuilt, and modified beyond design…
…and that is before we even arrive at the Periphery.
In-game a frankenmech is both a mech build from cannibalized parts, or one that has been repaired with sub optimal technology.
Many Periphery pirate mechs are a mis-mash of cannibalized mechs and systems equaling a frankenmech.
Imagine a Marauder that roll off the assembly line.
A hundred years later, lacking the credits, tech, and repair support what would it look like?
It is still a battlemech.
Perhaps the PPC’s have been replaced with medium lasers.
Perhaps it has no arms.
Sensors removed, staring down armor sights.
Armor repaired with primitive Battlemech armor, or even worse plasteel boiler plate.
In the tabletop game you might find yourself piloting a frankenmech if one is playing in a campaign.
I once had a Cyclops run through dozens of battles over a few years in game time- couldn’t keep up with the repairs, yet still needed to field it.
It in the end it had no arms, and was essentially a walking AC/20 with the other weapon systems being removed.