He probably was a 'good father' of sorts.
As lola commented, people are rarely so black or white. Villains are not instantly recognisable by their moustache and black cape.
By 'good father' I think the witnesses and QC meant he showed some demonstrable love for his children. He fed them, played games with them, built them a treehouse in the garden, changed a nappy on occasion.
Unfortunately Philpott was also an entirely self absorbed and selfish character who believed he should be able to get whatever it was he wanted.
It was this side of him that came to the fore when he decided to frame his ex partner for arson/attempted murder.
The poor children and their wellbeing never entered his mind at this point. They were less than an afterthought, they were just never a consideration in his diabolical scheme.
I'm not even so sure he is so devastated to have lost them as individuals in their own right iyswim. It is more what they represented to him, his virility, his brood of mini-me's, status as a 'father'.
So not a 'good father' in any deep or meaningful way but perhaps good enough in a practical day-to-day sense?