The tricky thing with evaluating Lepidus is that pretty much everyone at the time who was creating primary sources had their own reasons to bag on Lepidus, which means that later historians have a harder time trying to re-evaluate.
But I think Lepidus’ biggest problem is something he couldn’t really do anything about – the second Triumvirate was all about who was going to claim the mantle of Julius Caesar, whether that would be his military and political protege or his adopted son. Lepidus never quite had anything to match that connection, which meant that his political base didn’t go further than his army…and then he gave up his army in favor of being Consul and Pontifex Maximus.
And that’s what sealed his fate.