Yes, she was a bad mother for a wide variety of reasons, some of them:
Her belief that she is a superior because ''kindergarten '' work, and the complete lack of understanding that a regular aristocratic child and a Royal child which will end in the public eye for life are not the same.
The smugness of her being ''warm'' while others are cold.
She didn't understand the ''creation process '' of Royal children,-there must be a public persona and a private child, she mocked those that lived it before (other Royal women) which gave the world headlines ''William the Tantruming'' and ''William the Basher.''
As we see now, these get used against him, and would be even if he was the most zen person on the planet ,(media)
The ''uncles''
Affairs are one thing, usage of children as ''front'' for affairs are another.
See Spare and kids watching ''courtship '' of another uncle. (St. Tropez)
Carling ''teaching rugby'' and Hewitt , well riding , mostly Diana in hay of Baracks.
Even without the above, the way to early indroductions to ''uncles'', the witnessing of ''break ups'' and the general lack of stability would under normal circumstances give the other parent lots of leverage during custody establishment .
She was a egomaniac , she wouldn't ''improve'' her skills, because she did it ''right.''
She did things intentionally to create problems for the Monarchy it didn't matter if mocking protocol with mismatching gloves , or parenting the children as a form of one upmanship against the RF.
Of course the ''narratives'' her ''sainthood '' are something else.
The luurve, warmth, hugging , the kissing , the jumping on beds are clearly a successful ''polish'' and universally appealing to the emotivist mainstream but the reality is that Charles did similar things, with a difference :
-He didn't reach sainthood
-He maintained the public persona (mostly) towards his children when in public.
The childrens manipulation , the negativity and the (by today's standards ) parental alienation elements are clearly visible .
William succeeded at a certain point in seeing the ''intent'' behind her ''love'' love which would absolutely escalate when it was Charles turn to (have the kids) or do even basic parenting (see history 101, of ''superior '' vacations , ''superior'' outings as documented, always conveniently leaked, to outdo the other parent, leaked by her side)
Harry is now using these things against the RF. Contrary to popular belief he is aware of Diana's games (it was clear in school for him) but ...there is a Monarchy to attack , so it's useful .
Just like mummy.