This one has been debunked quite a few times, but it still comes up often. In fact, just recently, Wikipedia had Claremont and Byrne down as the co-creators of Dazzler.
Also, just recently, on Byrne's forum, a poster was confused that Byrne did not at least design Dazzler's costume.
Byrne informed the poster of what happened, which we know from previous Urban Legend installments (like this one and this one), that Byrne and Claremont had the character forced upon them after she had been created and her costume already designed, and in fact, the end result ended up extending the two-part Kitty Pryde introduction to a THREE-part story. It is quite a testament to the talents of Claremont and Byrne on Uncanny X-Men at the time that they integrated the character (who was created mostly by committee, with the costume being designed by John Romita Jr.) into the book as thought the pair HAD come up with the character on their own, so it is certainly reasonable enough for the confusion.
But it's not the case!