Ok so as someone who looks waaaaay to deeply into character design, but isn't profficient in drawing... I can say a characters design only has to match their personality, not nescessarily that is to be flashy. How good or bad looking they are should be a consequence of their actions and the appearance should help show that... On a more relaxed series it's ok to break that rule, but generally speaking good design should reflect who they are.
I'll post some art below to try and elucidate my point,

This is inosuke from Kimetsu no yaiba.
At first the fact he has a mask on makes yous question... Dafuq is going on? ANd sure it's a hot talking point, but consider the way the mask is portrated and you will see a lot of why it's there.
For one it covers the face (duh) but why? the face is an important part of the character as it's the main way to show emotion... So it is essentially saying that he doesn't care much for the emotions he shows and how people interpret him which is exactly how he is in the show being brash and charging into trouble.
Then consider what animal it is, a boar, know symbol of ferocity and toughness, it is a territorial animal who will fight you to the death if you get on it's bad side... Just like the character.
Then consider the rest of his atire... The lack of a shirt might seem like raw fanservice, but consider how feral this character has already been pertrayed, do you think maybe he just doesn't like how constricting the shirt feels? HELL he probably only wears the pants because people would kick him out of the city if he didn't... So it makes sense.
And the lack of shirt also reveals another thing, I mean, he is a sword fighter, so being naked can be seen as problem as he has no protection... and yet he has no scars... So you can be certain those blades are not just for show. and he is good enough to not need any armor.
We could also look at the dented blades and how they are made for ripping flesh as opposed to just slashing... But by this point you catch my drift.
Thing is, inosuke is not the main character of his story, Tanjiro is, i picked him to show how much even a 'hidden' character can reveal with character design alone.
But what about protagonists? Sure it can feel like a lot of main characters are 'cut from the same cloth' the generic isekai protagonist with black hair with almost 0 features, or maybe the 'monkey king' type of protag who always hast this goffy smile and never takes anything seriously, or maybe the wide eyed and kind protagonist like tanjiro (kimetsu no yaiba) or midoriya (boku no hero) can feel similar.
But those tiny diferences in design can make all the diference as they show how that character is unique when there are others like him,
As a bonus note... Not all protagonists are made to make you mirror off of them... It is common for the isekai protagonist to do that because of the nature of (most) isekais as power trip fantasies, but that is not mandatory.
A main character can look AND ACT completely different from us and still be interesting to follow... I mean... There is an actual manga about a guy who has A GUN FOR A HEAD! I'm pretty sure no one can relate to that.
And there are main characters that are not made for you to like them, so their design can often be featured in a way that makes them particularly mean or creepy... And they are still the MC.
So in short, the MCs design needs to match the tone and setting, there is no tried and true way of doing it other than to look into those details with a scope to try and find the appropriate elements you want to incorpate into the design.
The way to make a charaters design interesting is not nescessarily to make it look as unique as possible, but instead to lead the reader to ask questions about them and to get those answers to them later in the form of plot.
I also purposely avoided using the charaters of my story as I didn't want to feel like i was promoting xD