They didn't seem all that underrated to me, they were all just weak.
Buggy was weak because he got beaten with one attack. People disagree with this because of Nami stepping in, but let me tell you: Not being able to break a damn rope and WEAK GIRL stepping on you shows you're weak. If they fought again, Luffy would do the same thing (launching luffy's body instead of just attacking it).
Kuro, despite being some big bad pirate was still a weakling. Luffy beat him in a matter of minutes after he got a hold of him. Maybe he would've fought a little better if he had trained in those three years.
Krieg, the third of the East Blue weaklings only had the advantage because he broke apart the Baratie. If they had been on solid ground, Luffy would've just run up to him and broke his armor. It doesn't matter how many weapons he had, Luffy still shook them off like they were nothing (like the deadly… POISON SUCTION CUPS! He had to get 'em off, quick!) and of course, after Luffy broke his gold armor, Krieg was absolutely nothing. Another easy win for Luffy.
And now we go to Arlong, who I've always thought was the most overrated of the East Blue pirates. Big bad Arlong barely even DID anything to Luffy, he bit him an astounding two times (three if you count his Shark on in Nami's room). I've always thought of that fight as a complete joke, since it seemed more like Luffy was playing until the very end. Obviously, Oda had to stretch the fight out more, so he added Arlong's teeth (the whole taking them out and using them as weapons thing). When Luffy got serious (and majorly pissed off) Arlong died in one attack (Axe).
The East Blue pirates I feel are underrated a bit, but not much. Luffy never had much of a challenge until he met Smoker in Loguetown. It always annoys me when people say "Well Lucci was weaker than Don Kreig" because that's simply overrating otherwise weak characters.
Note, the Lucci < Don Kreig thing was an example, I don't really know if I've seen that particular one.