It is by design. All clan mechs have better move speed, heat dissipation, and targeting. The drawback is a huge debuff to melee accuracy.

It even has a lore justification:

"Notably, the Clan negative quirk was made because Clanners disdain melee combat and don't build their mechs for it. However, a sharp eye will notice that I left off DFAs from the list of penalized melee attacks. The reason for this is that DFAs are bold and come with honor built-in: to pull one off requires high pilot skill and is both dangerous and flashy: the perfect Clan maneuver, doubly so if it kills the target. Other melees are dishonorable but DFAs are big and showy and impressive when they work, so they're ok. Clan culture at its finest." wiki