Your own self-confidence issues are causing you to see the cosmetic negatives of Grendel far more than the positives. Warframes are meant to be easily distinguishable and have gimmicks. Other games with large casts make their characters have different voices or make them different races and species.They usually have a theme like "Cowboy" or "Scientist". But everybody in Warframe is just a android/robot person with a modern-art-esque vibe which is barely tied to real life concepts. So when I say "the fat guy who eats people" and you know exactly who I'm talking about, that's actually an accomplishment. Not a single warframe has ever been some real-life empowering figure. Equinox could have been a trans icon in the game. Nezha could have been a gay icon. Hildryn could have been a lesbian icon. Actually, they try very hard to make you not associate the frames with those concepts. Because warframes aren't people, they are just parodies of a single concept. Instead, they make the characters play to their barely existent "personalities". Equinox is Equinox. Nezha is Nezha. Hildryn is Hildryn. And Grendel is Grendel. The big fat monster who eats people. You =/= Grendel. If you happen to be fat in real life, cool. But you're not eating people. You're just a normal person living their life like the rest of us. Let Grendel be a parody of a fat monster that eats things. If you feel empowered playing a fat frame, then cool. If someone makes fun of you for playing a fat frame, then they are actually more insecure than you are because they are literally bothered enough by 1s and 0s to insult the actual human behind them. Let them be miserable idiots and keep on having fun playing the fat monster that eats people. Fat haters let fat people live rent free in their head everyday. Don't let them live in yours.
Let me say one more time. You aren't Grendel. You aren't the fat people the media makes fun of. You aren't the butt of anybodies joke. You are another person trying to live their life the best that they can. I really hope that you're able to gain the self confidence that you deserve. If you don't feel comfortable playing a character like Grendel in his current state, I perfectly understand. But I hope one day you stop seeing yourself in these media caricatures of fat people, and start seeing them as silly jokes, many times cosigned by fat people themselves.