The feudal society of Westeros is composed of more oaths and bonds than just that of king and subject.
Westeros's power structure is very complex. The (at the time of Robert's Rebellion) Targaryen monarchy rules the whole continent from King's Landing. Seven great houses owe the Targaryens fealty -- Houses Stark, Tully, Arryn, Lannister, Tyrell, Baratheon and Martell. These great houses then command the fealty of various lesser and cadet houses (the Karstarks or Umbers, for example) which themselves draw on various knightly manors and holds.
If you want, you could imagine this as a roughly pyramidal structure-flowchart, with the Targaryen king at the top and successive layers of aristocracy beneath him, with vertical bonds of fealty connecting the layers.
However, this is just the skeleton of Westeros' politics. On top of that pyramid, all Houses but the Starks (and the Greyjoys, but I'm mostly ignoring them for mainland politics) owe some degree of loyalty to the Seven and the High Septon. Were the High Septon to declare holy war (on whom? I dunno, just a hypothetical), all knighted Westerosi would face some significant pressure to raise their banners.
So, now you can imagine lines drawn from a separate entity, the Faith of the Seven, to almost every node in that pyramid.
On top of that, various Houses are constantly intermarrying and cementing alliances. On the eve of Robert's Rebellion, Houses Stark and Tully were about to be bound together with Brandon and Catelyn's marriage, and Stark and Baratheon with Lyanna and Robert's marriage. After the Rebellion, it is the union of houses Baratheon and Lannister which defines Westerosi politics.
So, now imagine just a whole mess of lines representing marriage bonds between nodes in that feudal pyramid.
And there's one last intangible element to the whole system: personal honor. Even taking into account the back-stabbing, dishonorable nature of the "game of thrones", personal honor will still define power politics, and in a sense underlies the whole system of fealty to liege-lord, the Seven, and family. By naming Jaime as a member of the Kingsguard, Aerys may not have violated any of the oaths and obligations between him and Tywin Lannister, but he nonetheless gravely wounded Tywin's honor by robbing him of his heir. This is arguably the deciding factor in Tywin's betrayal in the late days of the Rebellion.
What's clear is that almost every nobleman or -woman in Westeros is honor-bound to multiple entities, some of them conflicting. The liege-lord relationship to Aerys Targaryen is just one, and though strong it is not exclusive.
So what makes Jaime a pariah, and Eddard & Robert heroes?
Robert's Rebellion started with two mortal wounds to the honor of Houses Stark, Baratheon and Tully by the Targaryen monarchy. Rhaegar Targaryen abducts Lyanna Stark--Robert's betrothed and Eddard's sister--at the tourney at Harrenhal. Strike one: violating the bond of marriage between Stark and Baratheon. Brandon Stark, heir to House Stark and betrothed to Catelyn Tully, immediately rushes to King's Landing to demand justice. He and his father Rickard, Lord of House Stark, are instead murdered by Mad King Aerys. Strike two: violating the bond of marriage between Stark and Tully, and killing House Stark's lord and his heir. In the honor-based world of Westeros, there really aren't worse crimes for a liege-lord to commit. Even if Eddard and Robert weren't grief-stricken and enraged, they'd practically be honor-bound to respond. Add to this the fact that Aerys was known to be insane and cruel, and you have the recipe for a righteous rebellion.
Even in these circumstances, however, we find characters torn between their allegiances. Stannis Baratheon only barely ended up siding with his brother over his king. In the tangled web of allegiances he found himself in, honor-obsessed Stannis still found it more honorable to break his oath to his king.
Jaime's situation is different. The whole point of the Kingsguard is that they have only two responsibilities: to the King, and to the Seven. (And if it really comes down to it, the King wins out.) They can't inherit titles, they can't father legitimate children, they can't wed. They are supposed to be the pinnacle of knightly honor and chivalry. And in this sense, it almost doesn't matter to the victors that Jaime served their purposes. Barristan Selmy was more "honorable" by resolutely serving the Targaryens even through Aerys' madness. Arthur Dayne was more "honorable" even as Eddard killed him to 'rescue' Lyanna. Jaime violated his one vow in life, and in doing so sullied his honor in the eyes of everyone in Westeros whose position benefits from this honor-based system.
But did the author or any character attempt to resolve the blatant pettiness of holding a grudge under such circumstances?
That this bothers you is intentional. Especially with the first three books, George RR Martin is writing a subversion of medieval fantasy and chivalry in general. Jaime Lannister is an insufferable "smarmy jerk", as you put it, and yet his most courageous act--killing a king he knew to be an insane murderer, who intended to kill thousands more of his own citizens with wildfire--is the act which makes his "honor" as good as mud in the eyes of Westeros. This should make you question that value system. This, and, well, approximately a thousand other things which happen over the course of the series. Westerosi honor is bunk.
