In all fairness to Lepidus he is generally thought to have been pretty competent and an important ally for Octavian and Antony, but because he ended giving up his legions and taking a backseat role in future events, as well as the fact that his motives and the amount of autonomy he had in many of his political moves is questionable he always gets a bad rap. I mean, he ran Rome while Antony and Octavian were away fighting the Liberators War (not as glorious maybe, but no small feat), in the course of his career he became Pontifex Maximus, he became consul, and yes he was a member of the Second Triumvirate. Really in his personal life and career he did great as a politician and a commander, however in the end he did not make as large of an impact on world history as his two colleagues, but that does not make him incompetent, it just means there are fewer reasons to discuss him as much in mainstream history.

(By the by, is your username a GoT reference?)

Just to add a bit to /u/cleopatra_philopater:

Lepidus was a member of the gens Aemilia, a upper tier aristocratic Roman family. He had the social rank to be born into certain early career postings, and from there became an associate of Caesar. He was "elected" praetor in 49 BCE and was the de facto governor of Italia while Caesar was fighting Pompeius in Greece. In that position, he supported Caesar's brief initial dictatorship and then consulship. In this way, you can see how Roman political relationships were often imbalanced but also reciprocal. He was then sent to Spain as "proconsul" and did well for himself there as commander and diplomat. He was twice named Caesar's magister equum ("Master of Horse," the 2nd in command behind a dictator), including an appointment a mere month before Caesar was assassinated.

Lepidus was actually, in many ways, as relevant and powerful as Marcus Antonius in 44 BCE. He was, upon the murder of Caesar, more in the dictator's confidence than Antonius. Immediately after the murder, Lepidus moved forces into the city to maintain (or perhaps restore) order. He was in a powerful position, given his former position as "governor" of civil war Italia, and the following day both he and Antonius spoke in the Senate. Lepidus was named Pontifex Maximus, a clear nod to his auctoritas in the emerging political situation. When Sextus Pompeius (son of Pompey) started making trouble in Spain shortly after, it was Lepidus who was sent to deal with him. He had experience there, and of those in the capital capable of wielding imperium at this point, everyone clearly preferred Lepidus. He quickly negotiated an agreement with Sextus Pompeius and subsequently wound up administering both Spains and Gallia Narbonensis -- a very powerful position.

One problem for Lepidus was that he had not spent as much time with the legions as Antonius, especially the Caesarian core legions. He was a politician more than a soldier, in a time when those two were not necessarily mutually exclusive. I won't go into the details here, but at one point Antonius was attempting to displace Brutus in northern Italy, and Lepidus (in Gaul) was asked by the Senate to oppose him. Lepidus prevaricated -- he probably privately wanted Brutus annihilated for murdering Caesar, but he also did not wish to go against the Senate in support of Antonius. He managed to do nothing while Antonius was defeated at Mutina, then when Antonius moved towards Gaul with the remnants of his forces, Lepidus tried to negotiate with him. Let me play armchair triumvir here and suggest that negotiation was a huge mistake. He had the opportunity to destroy Antonius in the field, but instead he allowed the two forces to "mingle" under truce. This was disastrous, because many Lepidan soldiers knew Antonius well and "defected" to Antonius. It isn't clear from the sources what went on here, exactly, but this was the beginning of the erosion of Lepidus' position. I'm not sure what legions exactly were with Lepidus in Gaul at this time -- maybe somebody can chime in with that. It is possible that Lepidus did not trust his forces to fight Antonius on the field, and so tried his next best option. It is also possible that Lepidus planned to "join forces" all along (but I doubt it). Antonius had been loudly promising fat discharge pay for those who followed him, but I don't think that was the main motivation. He was just more charismatic, more a soldier, and by this time in the Republic the mercenaries now called "legions" tended to follow those they thought could win.