Is Karma underrated? I denied Karthus pretty well during the laning phase, used my Leona masteries (0/21/9) and it worked out very well... My shield and R+Q healing was enough to keep me topped off the whole time, and I got some good farming done with the help of wards to alert me of their sneaky Teemo (who actually said "Nerf Karma" because I shut him down midgame). After we pushed enough turrets I transitioned into support and let my team farm.

Damn, that Sion fed hard, and your Tryn snowballed out of control, it seemed.
If you wanted to build a true support Karma, Heal+CV or Flash+CV and rock the Gold/5 items. Running 21 in Utility on any support is very nice for the CDR, and CDR boots generally aren't something I build ever on any champ, unless they have a steroid that prevents any sort of disable, Gangplank's Remove Scurvy, comes to mind. Merc Treads or Ninja Tabi if the opposing team doesn't have any sort of solid CC should be the way to go.
I use to play a lot of Sona bot lane with a carry, and generally runed for movement speed and defense, make sure you always have a couple of wards and are warding, and if someone is missing, you should let you carry know not to push the lane too hard and just have them farm. You shouldn't be taking CS.
I don't think Karma is underrated, she's just outclassed by other strong supports and or AP carries. Soraka, Sona, and Taric have way too much utility and sustain to protect a carry over Karma. She's not a bad champion, but is one of those champions where it's hard to place her in any team comp.