This has never happened to me.
But when people (irl) look at my art and go: "Oh so you draw anime!!!" It's...kind of irritating because that's not what I want my art to be. So yeah, I kinda get how it feels.
I'm sure that a lot of people saying this kind of stuff really don't mean any harm and thinks of it as a compliment, comparing your work to something they like. They might even just want something to say in the comments. To the creator, it kind of sounds like they're accusing you of not being original, or even copying said work/franchise.
Of course we're all likely to be defensive about our comics and art- we put a lot of work and time into it, and hope to be recognized for that.
I don't think this situation has a person in the right or wrong. The commenter isn't trying to be offensive or cause harm (unless they are directly attacking you, in which case it's an entirely different situation), and the creator has a right to feel upset if this isn't what they hoped people would see their work as.
With all things, there's a line to be crossed. If a commenter starts harassing a creator on perceived similarities even after being told to stop, it's crossed the line, and they're in the wrong. If a creator starts attacking any commenter pointing out similarities and getting into a huge fit over it, they've crossed the line, and they're in the wrong.
We all probably react differently to this kind of thing. Heck, some people might be very pleased to be told that their art looks like anime. We can't control how readers will respond to our comics, and they can't control how we want to present ours. It's about communication, I guess, and what creators and readers both need to understand.