So? One voice alone can't save a dub. For example, Bleach had one or two decent VA's but the main characters VA was fucking terrible, and the scripting was awful (seriously, translating reiatsu literally into the words SPIRIT PRESSURE?) so the dub overall was absolutely abysmal.
So yeah, Mugen was pretty good but the rest of the dub was either meh, or bad (Fuu is SO obnoxious-sounding).
Well Fuu was only 15 or 16, so I'm sure the voice actor was directed to sound loud, energetic and annoying. I'd hit it.
The only lame voice actors in Samurai Champloo were like the one-timers like the "mafia boss" in the first episode. Things are always better in their native language though, because unless the animations are designed for English dialogue there will always be some iffy dubbing.