That is a fantastic question!
Linda Loman is not fully, nor directly, described as a parent in the play Death of a Salesman. Interestingly, her parenting style can be inferred from the way that she treats her husband, Willy, and from the way she acts towards her now-adult sons.
The reason for this is that Linda is a conservative woman whose position in society was to nurture both her husband and children. We know that Linda was the type of woman that would have done anything and everything for husband. Even at an elder age, she treated Willy like she would treat one of her sons: With compassion, mercy, and over-protection.
We can also infer that Linda, being submissive, let most of her boys' parenting in the hands of Willy. This way, the boys would follow their father as a role model. Equally, Willy revelled in his sons' successes in sports and especially in Biff's talent for football. For this reason, when Biff failed Math and could not move on to college (nor college football), the first person to whom he went for counseling was his father, and not Linda.
We can also infer that Linda was not as actively involved with the boys as Willy was because, when Willy began to act strangely, Linda ensured that Willy was "protected" from any conflict with either Biff or Happy. Had she been more involved with her sons, she probably would have learned to differentiate her role towards her husband from her role in the life of her children.
Finally, we can also infer that Linda was a type of enabler. She enabled Willy to carry on his salesman fantasies, and she even took part of those fantasies by pretending that everything was ok. She was also an enabler of Happy and Biff for not putting her foot down and forcing them to get out of their comfort zones. Above all, Linda was a parent in denial: She saw how the dynamics were developing around her and she acted as if her life was quite normal. This is why, years later, she encountered herself in front of two worthless adult sons and one invalid husband to take care of.