>>2458 What part of "Good at what it aims to do" sounds like "Good for what it is"? Aside from good and it.
If my vacuum cleaner doesn't vacuum, it is, as you say, good at not vacuuming.
However, what this vacuum cleaner aims to do(or at least, what I aim to do with it) is SUCK UP MAH FLOOR DIRT.
If an manga aims to pelt you with images of moe and panties, and does not, then it is bad at what it aims to do and is good at not doing so. If, however, it pelts you with moe and panties(like To Love Ru) then it is indeed good at what it aims to do.
tl;dr U R A IDJIT!