Frankly, this is going to sound sexist, but, he's a guy.
By which I mean, Sarah's entire shock value is that she is not actually as dainty, pretty, pure, etc. as society would expect she would be, as a lady. Despite this, she's conventionally physically attractive though this doesn't have as much of an effect because again, her entire shock/humor value is depression/unconventionally slobbish for a woman material. Which has its audience, sure.
On the other hand:
Ronnie's actually a chubby person with really niche interests, and plays the depressed nerd angle incredibly close to reality, while retaining that humanity and nuance that Sarah and Owlturd their ilk lack. It's not some standin for life or reality that's harming him, if ever it's his own motivation that hurts Ronnie. Beyond him, Ronnie's faults are entirely by his own design and not by social stigmas. Again, the relatability is palpable and it's like laughing at yourself.
Plus, he actually knows how to tell a fucking joke.