There are a number of characters that were either introduced as or later achieved a certain amount of competence/respect then later seemed to be downgraded (either in respect granted by peers or by readers). Sometimes downgrades stuck, sometimes they didn't.
Elongated Man is one for me. He was introduced as a competent hero. Later he was highlighted as a very good detective. Yes, he was an attention-seeker - no disputing. But he was still intelligent and capable. Had a wife who loved him. Later the attention-seeking got amped up desperate and pathetic and he was sometimes an idiotic buffoon whose wife treated him with disdain. I really like his silver age self, and was very disappointed with how he was treated later. That's even excluding Formerly Known as the Justice League and the JLA: Classifieds with him, which I disliked very much for pretty much everyone appearing that I actually knew.
Ted Kord is another. In his solo series in the 1980s, he was a very smart, capable guy. More than a shade immature in some respects, but few would think him incompetent or unintelligent, even if he's a rookie learning the ropes. That changed. Not necessarily consistently, as he still shined sometimes, but definitely sometimes he was treated like a very dim bulb. And I think made more immature (but maybe that's just him and Booster hanging out together and bringing out the inner frat boys in one another).
Dick Grayson is a third for me. May be odd to list someone so prominent, but I think he used be more intelligent, more mature, more capable, and more widely respected by his fellow heroes than he's often shown as in recent times. Again, it's not all the time, but it's definitely sometimes.