Je dois admettre que les soi-disant héros n'ont pas totalement usurpé leur réputation...
I assume that « ils ont usurpé leur réputation » has a figurative meaning of "they do not merit their reputation", "their reputation is unjustified", or "their reputation is not all it's cracked up to be".
And now, with the phrase « pas totalement {not completely} » thrown into the mix, its precise meaning eludes me. This is just guesswork, but does the entire sentence mean:
I’ve got to admit that the so-called heroes have lived up to their reputation, after all... even if just a little bit.