Yes, time spent on TopCoder is more likely to help you in coding interviews in general than picking up new technologies.
I say this with some specificity to Amazon (I did ~200 interviews while I was there), but it would apply equally well to every other tech interview I've done.
Top Coder will change the way you think about problems (in terms of what primitives you'll bring to bear against them). I would say the same for mastering another framework that has a different model than those you've used before, but that both takes a lot more time and is nearly impossible for an interviewer to evaluate unless they've achieved the same mastery.
Algorithms are in some sense a least common denominator proxy metric for "can this person solve problems?" Usually this is followed by "can this person string three lines of code together and perhaps use a for loop?"
When these interviews go well it's usually a pretty accurate indicator that the candidate is technically capable of doing the job. The contrapositive is not always the case -- when the interview goes badly you're sometimes left with a nagging sensation that because you did a crappy interview you're going to miss out on a good candidate. That's just the way (many) hiring systems are biased, though -- it's better to say no to a good candidate than say yes to a bad one.