I've found the clothes (and other items) that I have bought at WDW - as well as the Disney store - to be of varying quality, but almost always quite good.
2 comments, though -
#1 - Remember that BABIES GROW SO FAST. So it is difficult to know how long their clothes last, because they don't get the opportunity to wear them often! I know other parents can relate when I say that I have plenty of baby clothes with the tags still on them (And we are on our 4th daughter).
#2 - That being said, clothes at WDW are so very expensive, IMO. A bib or onesie will be $12-$15 and up. It's tough (at least for me) to spend a lot on something that they may outgrow overnight! On the other hand, the Disney store can be quite reasonable. And their clearance is amazing.
Congratulations on the little one, and have a great time!