She doesn't coordinate with her teammates.
First of all, notice how Takeuchi designed Sailor Moon and her team: all of them have white bodices and bold colors on their skirts and collars. The costumes all follow the same pattern.

On the other hand, I've seen teams of fan senshi that mix white bodices, black bodices, gray bodicies, and colored bodices. Their primary colors are bold, muted, dark, and pastel. Silver tiaras are thrown in with gold tiaras. Then to top it off, sometimes they have widely variating fuku styles. The overall effect is always a complete wreck.

Then there are the ones who are supposed to be part of the Sol senshi, but have a costume that doesn't fit the Sol pattern. Naturally, they always look out-of-place.

Her fuku is too complicated.
As the senshi progressed in power in the manga and anime, their costumes did tend to become frillier and more complex. But even Naoko Takeuchi knew when to quit - while Eternal Sailor Moon's costume was pretty frilly, Sailor Cosmos's costume took a step back toward simplicity.

Fans, however, often keep piling on the frills and details until they look completely ridiculous, and all those details distract from the character herself. 

Her hair is too distracting.
Naoko Takeuchi gave her characters both natural and technicolor hair colors. Whenever the character's hair had an unusual color, it usually coordinated with her fuku. Fans, on the other hand, frequently show no such restraint. They may have blue-haired characters wearing black-and-red fuku, which is an absolutely ghastly combination.

Sometimes, the hair styles are so big, huge, and complicated that they're completely distracting. Most of Takeuchi's senshi had relatively simple, uncomplicated hairstyles.

She's got too many colors.
Each Sol senshi has three colors to her design: her skirt and collar color, her bow color, and gold. Even Super and Eternal Sailor Moon didn't deviate from this pattern - Super and Eternal Sailor Moon just had more gold than the others. However, many fans overload their designs with colors, resulting in a wreck of a character design.

She doesn't even look like a sailor senshi.
Many fans get so gung-ho in their original designs that somewhere along the way, the character no longer bears any resemblance to a sailor senshi at all. Now, Takeuchi showcased a lot of variation in fuku design - in Stars, the sky was the limit. But they all had one feature that unmistakably marked them as a sailor senshi: a sailor collar.

Many of these designs don't necessarily look bad in and of themselves, really.  It's just that without the collar, they don't look like sailor senshi.  This essay further explains why it's just pure nonsense to leave the sailor collar off your sailor senshi.

I hope you find this helpful.  Happy character creating!