I'm going on the assumption it's a job 1) I like, 2) want and 3) am qualified for. If it's my looks that put me over the top and give me the edge to get it I have no problem with that at all.
I think your crediblity...comes from within.
As long as you have integrity...and believe you are qualified for the job...but, it takes your good looks to get it over another applicant...then, I wouldn't worry about crediblity.