Avram, we’re taking here about the small minority of women who in-your-face do nurse in public. Not the common sense majority (including non-Jewish) that will seek a private area — and in the absence of that availability coupled with the absolute unavoidable necessity of nursing without waiting until in a more private venue, will ensure steps to hide their nursing.
The former is clearly worse than a short-sleeved woman. Just as a woman in public in swimwear is worse than a short-sleeved woman. Even though both are offensive to public morality there are surely some more severe violations than others.