Yeah. Abe Lincoln also suffered from depression but no one was institutionalizing him for madness. Or depicting him as a madman in popular novels. Mary Todd did have mental health issues, though. I think it's more that the "madness" is a simplification and an overstatement rather than an outright fabrication. Anyway, it's been more than twenty years since I read the Vidal novel so I can't remember how much he leaned into the "madwoman" angle rather than a more modern analysis.

Edit - Since he was pressing the syphilis angle he probably did make her as "mad" as possible, but I really don't remember. He did make guilt a major element of Lincoln's depression, so it wasn't the "damn that Mary Todd for having feelings and therefore interfering with the great man of history" attitude I feel like I've seen in other fictionalized depictions of Lincoln.