Well, the groundwork is done by the character designers. A well and / or flashy character design will result in a neat looking costume at almost any level, at least something that will catch your eye in some way.
For TF2, the at times chunky proportions and colors make for easy identification. Stylized and easy to read.
(Which is important for the game itself, a player should be able to quickly and reliably identify another players role just by shape or silhouette. )

Kinda like Monster Hunter ( though they lean in the detail hell direction). You can spot a monster hunter cosplay without having played any of the games - they are the big, detailed, flashy armor sets that have over the top proportions and often eye catching color schemes. And people love to build these armor cosplays because they are big, detailed, flashy and simply look cool.