Originally Posted by
kjn
One can always find exceptions, but I have the same impression as Green Ghost and Mistah K88. It might thus be of interest to do some statistics, using the DC wikia.
Since New 52 (Prime Earth), Harley Quinn has 337 appearances, and Poison Ivy 158 appearances. Both are quite healthy numbers. I saved both lists as text and ran a diff; Harley Quinn had 264 unique appearances, and Poison Ivy 85. In 73 issues both appeared.
As a control, I added in Catwoman, with 232 appearances, as she is associated with both of them. Compared to Harley Quinn, Catwoman had 194 unique appearances, and Harley Quinn 299 unique appearances. They shared 38 appearances. Compared to Poison Ivy, Catwoman had 177 unique appearances, and Poison Ivy 103 unique appearances. They shared 55 appearances. In 30 issues, all three appeared, so 43 are Poison Ivy and Harley Quinn only, 25 are Poison Ivy and Catwoman only, and 8 are Harley Quinn and Catwoman only.
Note: this counts every appearance, even simple mentions, flashbacks, or being present in a single frame, up to being the protagonist or primary antagonist. More granular data would require manual checking of every issue.
From this, Poison Ivy is about as likely to appear alone as together with Harley Quinn, while Harley Quinn appears three times as often by herself compared to Poison Ivy, and Poison Ivy also often appears in association with Catwoman. Harley Quinn is in comparison more independent of both. But Poison Ivy is still associated more strongly with the Gotham City Sirens concept than either Catwoman or Harley Quinn, with only 60 appearances without either of them. The matching number for Harley Quinn is 254, and for Catwoman 169, if I'm keeping my numbers under control.
So I'd say that both views are correct here: Poison Ivy still has a decent independent presence, but she is very often used together in association with either Harley Quinn or Catwoman, or both.