Compared to most other Targaryen men, he definetely came across as a saint. The ficitonal author was constantly full of praise for him. Him being a not so great father to his daughters is probably the worst thing about him that was mentioned. And even that was maybe a handful paragraphs in the multiple chapters about his reign. History books tend to be very kind to successful rulers like Jahaerys. I feel like there is more there. I bet GRRM has some twisted ideas on how he really imagined him to be.