depends on the genre and how 'crappy' it is
If it's a quick really funny humor, as long as it made me laugh real hard, then it doesn't need that really detailed awesome art
if it's a long form serious manga with human characters, it would bother me if the anatomy is way off and the art is too far from the quality of the 'real' manga I'm used to read
I probably wouldn't get past the first few pages to even know how good the writing is,
Unfortunately when it comes to visual medium, the art does matter, for me at least