One of my favourite lines to come out of the Build Fighters series was, "Gunpla is Freedom!". I would expand that to, "Mechas are Freedom!". Any builder/designer should be able to go as far as they want to go and it should be okay for anyone else to have a critical or dissenting opinion. I personally, have a lot of fun building and modding kits to match my own tastes and I'll be the first to admit that my tastes don't necessarily line up with what others like. You take the criticisms with a grain of salt, some are constructive and will help you to improve your craft and some are just chaff.
This is some of my handiwork, it's a MG Zeta Plus that I completed earlier this year it is probably my best work to date and at the same time I can admit that it is still pretty rough. It certainly isn't cannon and it is very much built to my own tastes, I have gotten plenty of both flak and compliments in person and on the internet for it.
To Everyone: Once more, I'm merely suggesting that it's okay to have an opinion even to be personally invested in something but you don't need to be personally offended by every single dissenting opinion. One last tidbit of advice just leave him alone, don't add fuel to the fire, don't harass or troll, just leave him be.