I wouldn't put that down to them "all looking the same" though. Tajiri and Muta did well for the type of roll they were in, I don't think WWE really wanted or expected them to achieve anything other than mid card success. At least Tajiri anyway, I don't know enough about Muta to know what was expected of him when he wrestled in WCW.
KENTA in WWE could easily work, it'd just depend on whether if WWE would want to really push him and if they fans would take to him. If it weren't to work out for him it'd have nothing to do with Japanese people supposedly looking the same. That's just ignorant and absurd.
Tajiri had Regal and Muta had Gary Hart talking for them, if they don't speak English, they don't get over. Being a good wrestler isn't the end all, be all of professional wrestling. You can't get people to give a shit about you, just because you can wrestle.
Let's see how many Japanese wrestlers went stateside and didn't get over? Tenryu, one of the biggest stars in Japan, didn't get over and he was pushed heavily in his babyface tag team. Inoki didn't do much either, a failed PPV match against Ali.
It's not ignorant to say that, it's being realistic.