Michael Kay seems to think so, from what he said on yesterday's broadcast (and Al Leiter didn't exactly put up a fight). Hughes' supporters, some of whom are here, point out he's the youngest starter in the league (except Felix Rodriguez?) and he's essentially gone from Double A to the majors.
Here's what I see: He can't blow anyone away with his fastball. The Yanks, or him, or Posada/Molina, have him not throwing much besides fastballs and curveballs, which means he is getting a lot of strikeouts on the curve. Well, when there are guys in scoring position and two or three balls in the count, you can't throw that in-the-dirt curve. The fastball then becomes closer to a batting-practice pitch than an out pitch, and you depend upon a batter's mistake or superb defensive placement and ability to save you.
But that's not overrated. If anything, it's misplaced expectations, in part created by the six no-hit innings against the Texas Rangers. Even with a B+ fastball, he's still got about a strikeout an inning, something no other Yankee starter has close to matching.
Plus, he's held up under the pressure and not felt compelled to rush or hide his head in shame because he gives up some runs.
What are the solutions to the wall, of sorts, that he's hit?
1. Start throwing changeups and give the fastball the illusion of a 95-mph heater.
2. Become more of a Maddux/Glavine location pitcher. Neither is a strikeout king, but both can get big outs when they need them.
3. Have a natural learning curve and become better through repetition.
The only issue? Time. Doing any of those takes time, as it should, and it probably won't bring magical results this year.
As it is, this Phil Hughes that only has displayed a couple of weapons and a mislocated, misused fastball is a decent pitcher. He's good enough to keep in the rotation and good enough to hope that he and the team find that better path of pitching.
As for him being an ace, Andy Pettitte has never truly been an ace, yet he's had two 20-win seasons, eight seasons of 200+ innings and a career 119 ERA+. I'll take something similar.

I really don't pay attention to what Kay says anymore because he think's Bruney is good because he throws 95+ mph. So the reality is that Kay isn't the brightest person and a egomaniac on top of it.
Secondly, sometimes Al Leiter's anyalsis is usually good but not always.
I totally agree, Hughes needs to throw more changeups and give that illusion of that 95 mph fastball. Hughes is a work in progress but I wouldn't call him overrated, yet.