If a player chooses to use this talent several time on a same target, are the effects cumulative ?

I would treat the result of Crippling Blow as a status effect, meaning if a target is already "crippled", it can't suffer the effect again.

Nope, I wouldn't allow it and the reason why is whenever you have stacking situations it's explained, like Disorient, Criticals, Sunder, they're all explained

Thanks.