This is the point that most cosplayers would argue is the reason they cosplay without great knowledge of the source material. Cosplayers are attracted to the design, the look, the aesthetics of costumes. Here’s an analogy to explain. A group of hobbyist go to the movies. The photographer admires the cinematography, the DJ notices the music, the writer notices all the holes in the plot, and finally the cosplayer notices the costumes. A movie can be bad, but a cosplayer can still appreciate the costumes. Just like how individuals can like designer clothes without knowing the designer’s history, cosplayers can like a costume without knowing the series.
The joy of cosplaying is not strictly linked to becoming the character and acting like them. Sometimes it is the challenge that the costume will bring. Conversely, people can be huge fans of series and have zero desire to cosplay the characters if the designs do not appeal to them.
They may not be knowledgeable of the series but they may be a fan of the artist [character/costume designer] instead. I know many cosplayers will make a CLAMP costume without a second thought about what the series is about. The style just resonates with them. People are willing to make obscure costumes from one-off illustrations in art books.