Well let's just get this out of the way: They are mentioned. Not as often as Uncle Ben or Gwen. But they are. Especially with the recent Scarlet Spider/Kaine solo title, the death of Ben Reilly was mentioned often.
But I think in the cases of DeWolff and especially Ned, you have to think of the relationship Peter has with these people. DeWolff is someone he knew only as Spidey. Yeah the famous "Death Of" storyline hints there was something "more" on Jean's end, but and sure, some writers liked her and used her. But she was just one of the go-to friendly cops that Spidey would go to for information. There was no type of personal connection or bond between the two characters. She also wasn't present for her death.
In the case of Ned, they weren't just not friends, but rivals. At times, they hated each other. He also wasn't present for his death and he was partially responsible for a lot of misery in Betty's life. So yeah, I don't think Pete not mourning Ned's death is out of character. In fact, it makes a lot of sense.
At least in the stories that I read. In Amazing #289 ( When at Ned's funeral.), Peter's internal monologue even states that Ned was a friend.