It really depends but as long as the finished product looks fine it doesn't matter imo (unless the intended goal was to make a near replica). For example the Death Star 10188 is an awesome model that focuses on fun proportions inherent in Legos and its a theme you'll find in other franchises like Blizzard's Starcraft/Warcraft and Games-Workshop's Warhammer series.
Personally I want my own MOC's to be somewhat playable and usable for minifigs like the Republic Frigate 7964 but thats if I ever get around to making any. I'm rediscovering this hobby and have always wanted to make the Outrider from Shadows of the Empire however it will not rival the UCS-caliber MOCs out there which isn't a bad thing if its not a goal. Currently I want to make sets that look like they're a part of the regular lineup but if I had a lot more money to spend I can see myself considering otherwise ;)
When it comes to critiquing other MOC's it should be constructive unless the creator specifies otherwise, and if they don't even want any they should make it clear in the original post. There's also factors like building experience, the intent of the MOC, etc. to take into account. Its generally courteous and respectful to start or include positives in a critique as well.