The clothes looked more appealing inside the store than on the racks..

The sentence is fine the way it is. You are comparing 2 locations: inside and on the racks outside. The parallelism is regarding 2 locations. Furthermore, "on the racks" is modifies "outside" so you are comparing inside and outside.

"..inside the store than outside the store on the racks" is fine as well, but there's no such thing as "direct parallelism" that you speak of. You can omit some words in parallelism if there's no ambiguity. For instance you can say "The clothes look better inside the store than on the outside" instead of "clothes on the outside of the store" because it is clear what you are talking about.

However, if you say "The clothes look better inside the store than outside", now there is ambiguity. Are you talking about outside in general or the clothes outside? We do not know. Therefore, you must at least say "on the outside"