SEED is Cosmic Era, not Universal Century. It's a parallel timeline and terminology may differ a lot.
In UC terms a Mobile Suit is a humanoid-type manned mecha. It's four-limbed because the arms and legs are not only for propulsion and handling of equipment/weaponry but because of the AMBAC system that allows thrusterless attitude control in space (a bit like a human may use his limbs to control his rate of spin without resorting to the suit's thrusters).
A Mobile Armour is not necessarily humanoid-shaped and it doesn't need to be. It's the size of a small ship with a massive Minovsky reactor, megaparticle cannons and even I-field generators. It's a different design and originally competing against MS when Zeon started its armament program.
The problem with MAs is that they're massive and expensive, and they're all one-of-a-kind prototypes. They are usually relegated to a few specialized roles. However, they're enormously effective when correctly designed and employed: the Apsalus could easily burn through a mountain range (and it was a COTS model) and independently target and destroy dozens of enemy MSs, while the Big Rang destroyed 8 Feddie ships and lots of GMs all the while refueling and rearming Mobile Pods. Neither of them were piloted by aces, I should add...
However, you can add a module to a big enough MS and turn it into a MA, like the Dendrobium Orchis, or have a hybrid tranformable mecha like the Asshimar you cited. The Asshimar is small for a MA but it's also very powerful for a MS.
