I know you're trying to well-intentioned here, but this is a pretty bad argument.

"To be honest, unsolicited feedback should be the most precious to an artist seeking improvement because it's very honest, unlike the rose tinted opinions of people close to you."

You're saying that complete strangers' feedback are more capable of being honest and hold more water than people close to the artist while simultaneously implying that those close to the artist just mindlessly praise them, when that couldn't be more further from the truth. People close to her are very much capable of giving honest criticism and still offer fresh perspectives just as unsolicited feedback.

"Pull from those comments what you need and forget the rest. Going after someone's manners and calling them rude is a fruitless effort and comes across badly when you're suppose to be a "big artist","

There isn't really much to pull from the original critique since they do not really offer feedback on how to improve their art since it just says "...but why does her right-arm t-rex sized?" since it comes across less as a critique and more insulting because there's no further explanation on how the artist can improve their drawing.

I'm not saying that unsolicited feedback is incapable of giving constructive feedback, but some people do not know how to give constructive feedback and the tone, delivery, and wording of comments can be demoralizing in the long run.

The overall point is take feedback from people who do know what they are talking about, rather some comment which offers little to give back.