It is all about the looks yes, but a buggy and truggy in stock form has distinctive features such as lower ground clearance, big wing shorter body posts, smaller and narrower tires, and shorter suspension travel. Now if you change these features on a truggy or buggy to resemble a stock "MT", you in deed change the apperance of it and characteristics.
The 3xxl is per definition a "muggy" i.e. it has characteristics from an MT such as ground clearance, suspension travel and fat wheels, and characteristics from a truggy such as center diff and possibly something else which I don't know about for the moment.
My custom e-maxx is per definition a muggy because it uses a center diff from a truggy, low CG chassis, and buggy diffs, but it also has characteristics of a stock MT such as big wheels, pickup body, "no-wing" and higher ground clerance. For me it is still an MT because if you would see it from a distance it doesn't deviate too much from what most people think a stock monster truck RC looks like.
The kraton how ever looks ugly regardles if it is an MT or muggy or truggy. The stock e-maxx for example look alot more dope because it looks like a truck with zero emphasis on competitive driving (bashing). The e-revo is ugly as well, the body looks like a camaro and the wing just look stupid as well as the small wheels. The summit how ever looks like an MT.