There is only one Tendulkar, and his name is God. :P
j/k
Anyway, about Mark Waugh. I feel the record speaks for itself. He was a good ODI player, moreso than Steve, but hardly a match in the test arena.
I never thought of him highly as a test player, but as an ODI player - great success. An average of 39 odd speaks for itself. Leave the 40s for the legends...if you can keep a high 30s average in his day, that's quite good enough.
He will probably always be remember as the Great Steve Waugh's brother (at least, I, unfortunately, see him like that) but that happens to the best of them, and just because he's overshadowed by someone greater doesn't mean he wasn't great in his own right.
And personally, I don't think his ODI highest score being higher than his test highest score says anything. If anything at all, it says that one fine day, Mark Waugh went beserk. That's it. Highest of 154 is also fine...you really don't need a highest of 375 to be regarded as a great - his average, while not spectacular, is quite good and the rest of his test record matches it. You can't blame someone for not being good enough.
Underachiever? I don't know...he did well...