(disclaimer: I'm not a fan of Townsend, I'm not trying to defend this specific company exclusively, this explanation applies to ALL fashion of this genre)
The reason why this doesn't look great is because it's not fitted properly ON PURPOSE. The truth is, a proper gown of this time period, no matter upper class or lower class (this purple one is a middle-class frontier America), is suppose to be fitted to the individual. The only way this company, and many others, can produce reasonably priced garb (without tacking on another $200, $300 in labor, sending the garment out, having it sent back, re-fitting, re-sending out, etc etc) is to fit it just enough that then YOU only have to do the final adjustments/taking in.
In the 18th century most gowns below royalty had no exact patterns; people would measure with string cut at the point of overlap to use to the cut materials and would roughly copy the shapes of dresses based on pictures and dresses they already had access to. Once the rough shapes were cut out, they were then pinned DIRECTLY onto the future owner of the dress and the needed take-in points were marked. Ideally, you are suppose to set the sleeve of the bodice/jacket while it's on the actual wearer. As a last note, there were no snaps, zippers and a limited supply of buttons; your stomacher would be pinned directly into your Stays (corset) every morning and then you would unpin yourself every night.
With all this in mind, the only modern dress makers that will be selling custom fitted-correctly garments of this era will be charging you A LOT more and will require more measurements from you and or you might have to go in for fitting.
Retro-actively in the 20th century we've created sewing patterns for these garments by studying the surviving ones, but often (as mentioned with the simplicity pattern above) they still will never fit you correctly if you hire out the work and have it shipped back to you.