I don't know what Joel S. was watching when he was making comments about Fonzie being a loser.
Fonzie was supposed to be older than the rest of the gang, but not by that many years. Perhaps it was because Henry Winkler was older than the rest of the cast that he looked, as you said, twenty years older.
Fonzie never dated high school girls. He knew they were too young for him. He had morals.
Fonzie being an illiterate high school drop-out? I don't know where you got that from. Fonzie had dropped out of high school when the show started, but one of plot points of the episode where Richie graduated high school was that Fonzie revealed that he'd been secretly going to night school to earn his high school diploma. He graduated with the rest of the gang.
Fonzie living above the Cunningham's garage. That was because he'd given up his own apartment to his grandmother after she'd been forced to leave her own place. He stayed above the garage for so long because he loved he Cunninghams like his own family. He essentially was a part of the family. In the last season, he did move out into a regular apartment. In the last episode he bought a house so that he would be allowed to adopt an orphaned boy he'd befriended. Gee...buying a house so you can provide a good home and be a good parent? Doesn't sound like a loser.
As well, Fonzie also worked several jobs at once. He was (or became) the owner of the garage he worked at. When Arnold's burned down, he put up money to help Al rebuild and became the part-owner. Then, he started teaching shop class at Jefferson High. He later went to a tough school and became the Dean of Boys, so he could help kids who needed guidance.
So, I think Fonzie was a cool character not because of his leather jacket, or motorcycle, or his prowess with girls. I think he was cool because he was a good person who was always willing to help a friend in need. Did you ever see the episode where Al wants to go down to Alabama to join a Civil Rights march? (This was a later episode when the time was the 1960's). Fonzie is concerned about Al's safety and goes with him to look out for him. Fonzie joins Al and a young African-American man in a sit-in at a diner. That doesn't sound like something a loser would do.
