You might think that it takes a 'thick skin' to be a designer. Certainly all designers face rejection and a thick skin would come in handy. I'd rather think that 'resilience' is the better trait. Being thick skinned is to emotionless and doesn't really appeal to me. It's ok to care about your work, it's ok to care about success (however you define it). When disappointment happens, it's better to be resilient, to be able to console yourself and move on. To avoid being disappointed at all is just futile.
So, in summary, it is helpful for a crochet designer if you are:
Not afraid to be yourself/original
Not-so-much a Perfectionist