Well I thought I'd just go ahead and break some ice here and maybe make a forum topic!
Anyway, are there any cartoon characters that you've noticed are super popular in rule34 and that everyone seems to get hard-ons for, but don't seem attractive at all to you?

My two examples have got to be Marge Simpson and anything that has Seth MacFarlane's artstyle. I've seen images featuring them with tens of hundreds of upvotes (mainly on xbooru, go figure I guess) and numerous comments talking about how sexy the character in question is, but for me they just don't get me going even with an artstyle change.

I don't really like Tracer from Overwatch. Even though she's the most came-on on Rule 34.

I've never really cared for Harley Quinn.

Yeah that's another one I've felt a little lack towards. I mean her outfit is hot and all, but she's just another basic brunette girl to me. There's a dozen other brunettes fictional or otherwise that people can swoon over easily.

It's the cute accent.

Somehow people really like Peach from Super Mario? I never got it at all. I also don't have much for Mercy from Overwatch. Or Elsa from Frozen. There are tons of them out there, way too much to count.

I agree with what most people have already mentioned. Harley, anyone from Family Guy or The Simpsons, Tracer, Ciri from the Witcher, and I actually had a conversation with my boyfriend about how popular Princess Peach is.

I was trying to understand why she is so wildly popular considering she doesn't seem to have much of a presence or personality, she's very cartoony/geared towards children, and she's always dressed rather modestly (as far as I know, I haven't checked out recent games). It's hard for me to understand why so many people fap to her when it seems like she isn't even remotely sexual. I don't know, maybe it's because I'm not much of a Mario fan.

There are plenty of characters that don't get my gears running but I can understand why people like them, I just don't understand Peach.

Someone finally preaching the gospel. The accent, forced ditsy personality, and general sluttiness in the more recent depictions just doesn't do it for me at all. Pretty much every other character that's been listed here I agree with not being overtly attractive sexually (Elsa being the exception). I'd add Lara Croft, Elizabeth from Bioshock (the nose and forced attention towards her chest kill it for me), Mary Jane Watson, and Asuna from Sword Art for me personally.
Edit: Jeezzzz, how could I forget Princess Zelda, Wonder Woman, Starfire, Raven, Ada Wong, and Jyn Erso? (New Star Wars? *Vomits*)

Oh the list I have. I can definitely agree with Marge and Tracer. Tracer's accent is hot, but that's about it for me.
Misty from Pokémon is the first one that came to mind. I know she was one of Ash's first traveling companions and the first female gym leader, but the other gen 1 GL's are way hotter. Maybe it's the attitude?
Raven from Teen Titans is another one. Call me a hypocrite for preaching goth girls are hot, but not liking Raven. I guess she's just way overshadowed by Starfire to me.
Elsa and Anna from Frozen are the last two I can think of, but I don't have much to say about them.

Harley has an accent? Wtf, what kind of accent?

It's a clear accent. Like Boston or New York or something like that. I honestly get the two confused.
Personally the accent's one of my favorite things about Harley.

Depending on what you're watching, it can be a way too bubbly and high pitched Jersey accent bordering on a weird nasal tone, a milder Brooklyn accent, or an equally as high pitched Brooklyn accent. Suicide Squad was the milder Brooklyn version, but in any of the old cartoons it's almost guaranteed to be the Jersey version. Point being, the high pitched and/or nasal voices and accents grate my nerves to the point I want to deafen myself so I won't hear it anymore. *CoughTracerCough*

Any character from the Sonic universe;Lillie,Cynthia,Zinnia,Serena and Dawn from pokemon (I kinda hate some of them,but...);Ruby from RWBY;Tracer and Mei from Overwatch.

Huh, weird. I've always loved Harley's accent, never really got to me. As for the character herself, it's original Bruce Timm Harley or bust! (Gods and Monsters Harley was bangin' too though.)

I will be honest, I'm not a fan of getting off to MLP characters.