Underrated in gameplay:
Baraka, Noob, Scorpion.
Baraka and Noob are great great GREAT when cornering an enemy, and Noob can zone pretty well and range greatly, and using his A,A,A combo (on 360) is pretty effective up close in really tight moments. Baraka's combos are relatively safe and using his spinning move helps trick players up very well. Scorpion, because he is easy to pick up, is underrated. I cannot count how many tough matches I won as Scorpion, just because he is reliable, fast, and can string long combos, even though every one goes for the same one which is not very effective with the lag from online play (the one with the jump kick teleport) I may post my combo from MK9 on here because it is easy, hits hard and high percent damage and is easy to hit online. I have never seen anyone do it online either.
Underrated in other terms:
Drahmin and Nightwolf...
Drahmin is an oni with a cool look and storyline (corrupt warlord damned in Hell now) and it sucks his moves turned out to be so cliche and ineffective beyond anything. I want to see him returned with cool moves.
Nightwolf is so powerful and unique, being the native american with a faith in Raiden despite he is not an asian character. He is a different type of character, with a cool personality (humble, angry at times, and powerful)
Overrated in gameplay: Kung Lao, Mileena, and Cyrax.
Kung Lao is fast up-close, but is pretty predictable from a distance. His teleports are easy to block or evade, and his hat throw (while can be guided) is still a little predictable as well. He is however, fun regardless and is powerful, I just think people overestimate him.
Mileena is extremely predictable after a while, and is more annoying than anything. She isn't too hard a hitter, and her speed is good but for how many people play her, I do not think she is that great a choice. Her combos are a little slow in exchange for fast specials, and leaves her kind of screwed against more brawler characters that can close distances fast. I do not find her fun to play.
Cyrax relies heavily on the bombs and is fast, and is used a lot. He is pretty high-tier in my opinion, but I find when you get passed that he needs to use enhancements and meter burning and his bombs are thrown a lot to set up moves and strategies (keeping pressure via the bombs) he is pretty predictable. As long as you are constantly thinking and moving, Cyrax cannot really match up very well physically up close or from afar...
Just my opinion. Every character can be played well, but you know that statistically there will be patterns. There are always exceptions.
Overrated in other terms: Reptile and Rain
Reptile is overrated, considering half his fans liked him as a human more than his current appearance. He barely speaks now in the games and is kind of popular for it, like Boba Fett was in the original trilogy. He is constantly being shit on by Midway and NRS and yet still finds a way of being loved.
Rain is all based on a joke, references all used to create one character. He was requested greatly for MK9, yet in online play did not last much longer than a week after his release. He is not played often and just seems like he is... there. He has almost no big or critical effect in the MK plot, and when he does, it is not made very evident or important.
That being said, all MK characters are equal regardless.