Have any of あなた been told in a PM または review that your fanfics are way too serious and then あなた feel bad about it?

No. And I would never feel bad about it. If I don't feel like 書く about light subjects, then I don't. Simple as that. Besides, 'way too serious' is subjective. あなた can't provide every reader what they want.

I was told once that I used eyes to much to 表示する emotion and that I should cut back on mentioning eyes in my work because the reader thought it was bad writing. Actually, the person was likely someone I critiqued who decided to create a 靴下 puppet account in order to attack my work, but his stuff was far from legitimate. As for how I felt, I didn't feel upset about the situation because I knew that the reader wasn't write.

That said, I have told one person not to take their 書く as seriously as they did because they focused on not skipping the boring parts and the grammar/spelling rather then the actual plot line, story and characterization. I told them to refocus and balance out a bit.

Well, I once had a baby killed off in a Full Metal Alchemist fic of mine, and this girl flipped her shit. I was mostly irritated. But fanfics can't be too serious. It's your imagination, unless, maybe you're a little too serious または out of character of the characters you're 書く about.