Michio Kaku is not necessarily the best in his field, mediocre at best, but he has the biggest voice.
I agree. But this isn't really news; This is how it's _always_ worked. The public is not going to figure out the merits of your scientific achievements on their own, and then give you attention that's proportionate to that. It's the same as in any other area: You have to market yourself.
Linus Pauling was arguably the most famous chemist of the last century. But he wasn't actually that important. The quantum-chemical contributions he made were in reality on-par with those of Mulliken, Hund and Slater. Many would say Slater should've shared in his first Nobel prize. But it was Pauling who wrote "The nature of the chemical bond", it was Pauling who popularized the subject, it was Pauling who was the bigger educator and public figure (which was not limited to chemistry). Richard Feynman was one of the most famous physicists. And while his contributions are also beyond question, they were arguably not a lot larger than those of, say, Murray Gell-Mann, who is nowhere near as famous. Because Gell-Mann was not a big educator. His popular-scientific books didn't sell anywhere near as well. Dirac was as important as Bohr when it came to quantum theory, but he wasn't anywhere near the popular and public figure Bohr was. And so he's also less known.
What bothers me about Kaku isn't the fact that his fame is disproportionate to his scientific contributions, or even the fact that it leads people to think he's a greater scientist than he is. What annoys me about Kaku is his propensity to comment on stuff that he doesn't know much or anything about. For instance, his statements on evolution, which were harshly (but justly) criticized recently by PZ Meyers. Or his commenting on the Deepwater Horizon spill, the Fukushima diaster (which he, IMO recklessly, called the worst diaster second only to Chernobyl, even though it's far from clear that it'd be worse than Three Mile Island or Windscale at this point, and certainly several orders of magnitude less severe than Chernobyl). And now we have him commenting about Moore's Law, even though he's not a solid-state physicist.
I suspect he's letting his ego cloud his better judgment. It's not uncommon - the aforementioned Pauling, for all his scientific merits, had a whole bunch of bad, crankish ideas in areas outside his field (nuclear physics, vitamin megadoses, anesthesiology). I don't believe at all Feynman was the humble guy he tried so hard to make himself out to be, but to his credit, he was quite respectful of other fields and did not have that propensity to make himself out to be an expert on things he didn't know much about. Of course, there's also the possibility that it's not about Kaku's ego and that he just genuinely doesn't actually give a damn about educating the public, and is more interested in just getting attention for himself. But I'm prepared to give him the benefit of the doubt on that.