・Some politicians are no better than they ought to be.
I interpret the sentence above as 'some politicians are even worse than we expect them to be as politicians.'
Is my understanding correct? 
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I think your sentence is more an 'addition' than an 'interpretation'.

The original: we shouldn't expect some politicians to be any better than they are (?)

It's what we call a "loaded sentence"!
 You mean the original, not mine, doesn't really make sense?
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I should have said 'a few might be even worse than we expect them to be as politicians' instead, by the way.
Or does it mean something like 'some politicians never go beyond our expectations'? 
>Some politicians are no better than they ought to be.
I find this original only in a Japanese blog, and it is confusing.

Yours makes sense.
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OK. Thanks, MH! 
Just out of interest, let me pick up this old thread.
PhilipIt's what we call a "loaded sentence"!

What does 'a loaded sentence' really mean here? Does the sentence 'some politicians are no better than they ought to be' not make any sense at all? If not, then why not?
It's a colloquial expression.

I think it orginally meant that a young woman was not overly particular in bestowing her sexual favors. That is to say, it was a genteel way of saying "She sleeps around." However, with a woman's sexual promiscuity no longer regarded with the same degree of horror as it once was, it's now migrated to a more meaning of "They should behave better than they do."

See this: ... baffled by the phrase `She's no better than she ought to be.' If, as he says, his own mother was not troubled by speculation about it, might it not be because she understood perfectly that it meant, `She ought to be better than she is'? As a matter of fact, there is a common variant, `She's no better than she should be,' which by the same token means, `She should be better than she is.'

from http://findarticles.com/p/articles/mi_qa3724/is_199802/ai_n8786608/
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