You are a:
Turtle/ Tortoise (I know they are different but bear with me)
Some think I'm fashionable and wear me
Turtleneck sweaters and turtleshell spectacle rims
Some say I'm slow yet I'm faster than a showoff
The turtle and the hare (technically tortoise)
When I'm down, my friends tell me that I will eventually get back on my feet, but that's just cruel
A tortoise upended onto its shell has a lot of difficulty getting back onto its feet, and leaving one in that position is rather cruel
Sometimes I dress up like those sneaky Japanese
Ninjas as in TMNT (Teenage Mutant Ninja Turtles)
I might be robust and durable, but I tend to not stick my neck out unless I really have to
self-explanatory