I've actually done well with Karma top, if I don't get to be the support role. As against Jayce, I believe she can have the upper hand in some situations. The last game I remember going top with her was against a Xin, and I did really well. It's all a matter of itemizing right on the spot. If they're AD I usually rush a Glacial Shroud to a Frozen Heart.

The thing about Karma toplane is that her MS is terrible and any jungler who knows anything about Karma (oh thank god that is only 0.00001% of junglers) would simply camp your lane or gank you if you even pushed to the river. If the enemy jungler has any farm of CC (Maokai for example) then you're pretty much a free kill every time.

Camp lane til she uses up her mantra.
Gank.
No shield nuke/clutch self heal.
Free kill Karma.

In all honesty her safest lane - and one she outtrades most characters in - is mid. Least I've only had problems against Cho'gath (CD on mantra isn't low enough to not allow him to sustain), Lux (she can stay at a safe distance and poke you easily with E's), and Kennen. Although Kennen was likely more or less my poor experience vs aggressive Kennen mids.

Xerath is probably a similar issue as Lux but he doesn't see much play.