Actually, considering that Ivy has been part of team titles like Gotham City Sirens, Birds of Prey, Suicide Squad, shared titles with Harley and even had her own mini-series, young adult graphic novel and multiple one-offs, she and the other two Sirens are among the top 5
most used Batman villains, along with Joker and Penguin, according to this website which does a pretty good job of counting as many appearances as possible made by each character in comics:
https://comiccruncher.com/
The Gotham City Sirens are the only 3 villainesses (if they can even be called that at this point) who appear in the Top 100 (I'm not counting Emma Frost as a villain as she has firmly been on the X-Men's side since the 90's). After the Sirens, the most used DC villainesses are Amanda Waller and then Talia Al Ghul. Cheetah is the 6th most used DC villainess but she's not in the Top 200 most used characters. Marvel has a worse record with their highest used villainess Mystique appearing out of the Top 100 at #120, with Black Cat and Moonstone being the only other two female villains in the Top 200, and Lady Deathstrike missing out at #252
It is quite the terrible showing for female villains as there are only 9 who appear in the Top 250 most used characters, 6 of which are from DC:
1. Catwoman (Rank #56) DC
2. Harley Quinn (#80) DC
3. Poison Ivy (#94) DC
4. Mystique (#120) Marvel
5. Amanda Waller (#130) DC
6. Black Cat (#137) Marvel
7. Moonstone (#171) Marvel
8. Talia Al Ghul (#182) DC
9. Cheetah (#216) DC
https://comiccruncher.com/