They're popular, yes, but I'd hardly call them iconic.
I think some of us aren't clear on the meaning of the word 'icon'. An iconic character isn't just a character that you like or one that is popular. Some characters, whether you happen to like them or not, are especially memorable or historically significant or interesting, and therefore everyone knows them and they in some way set the standards for other characters and shows which came after them. Those characters are icons.
Example, people aged 4 to 94 know who Mickey Mouse is. You could go to a mud-and-bamboo hut village in the jungles of Undi Barundi and show them a picture of Goofy or Popeye and they would know who they were. That's an example of an iconic character. Stewie Griffin is popular now, but are people still going to be talking about Stewie 30 years from now? Are Stewie, Beavis and Butt-Head instantly recognized by all walks of life, regardless of age, social status, geography or ethnicity? I'm thinking not. Therefore, those aren't examples of iconic charcaters.
To reiterate: one's personal favorite characters or hot right now flavor-of-the-month characters =/= icons.