Insulting the intelligence of someone with an opposing opinion is generally considered bullying by most standards. Pointing to people who do worse things than you is a lame attempt at deflection. Why not try taking responsibility for your actions instead of taking pot shots at people who disagree with you?
I agree with the second sentence of your post but not the first. I've never heard a definition of bullying that didn't include actions that are unwanted and repeated. Personally, I wouldn't consider it unwanted for someone to tell me my opinion was stupid, although if they didn't explain why then I wouldn't give their opinion any credence.
I've been called full of shit and mentally ill by posters on FSU. While unkind, I wouldn't ever consider that bullying but rather just unwarranted rudeness and offensiveness.
As to whether others would be more bothered than I was, sure some would but some wouldn't. However, throwing out the word "bullying" in instances in which it's pretty clearly not bullying trivializes actual bullying. I work in a high school and, in the state in which I live, there have been numerous high-profile examples of suicide and other extremely negative consequences of teenage bullying. Calling someone's opinion dumb on a contentious online forum is not the same thing.
This might seem like a minor point, but as a person who cares about fighting against bullying--and other negative stuff like heteronormativity and the continuing popularity of Guy Fieri--I personally woudn't throw out accusations so hastily.
As long as I'm on this soapy tangent, I will add that the continuing trend of men discussing whether women (or "females," a term that sets off a bunch of alarms for me) are stupid and/or fat is really unsettling.
Still rooting for Tonya. And Adam. And for Mirai to prove all my snark about her expressionlessness wrong.