Dumbledore may have had other reasons for selecting Ron as a prefect. Consider that it seems that Harry should have been the obvious choice as Gryffindor boy prefect:
Standing where the twins had left him with nothing
but a guilty weight in the pit of his stomach for
company, Harry caught the sound of his own name.
Kingsley Shacklebolt’s deep voice was audible even
over the surrounding chatter.
“... why Dumbledore didn’t make Potter a prefect?”
“He’ll have had his reasons,” replied Lupin.
“But it would’ve shown confidence in him. It’s what
I’d’ve done,” persisted Kingsley, “ ’specially with the
Daily Prophet having a go at him every few days...”
The twins’ heads turned in unison and both of them
stared at Harry.
“We thought you were a cert!” said Fred in a tone that
suggested Harry had tricked them in some way.
“We thought Dumbledore was bound to pick you!”
said George indignantly.
Dumbledore himself later admits that perhaps Harry should have been prefect:
“I feel I owe you another explanation, Harry,” said
Dumbledore hesitantly. “You may, perhaps, have
wondered why I never chose you as a prefect? I must
confess... that I rather thought... you had enough
responsibility to be going on with.”
Now, what other reasons could Dumbledore have had for appointing Ron as prefect? Well for starters, there wasn't much competition. Throughout the series we only ever see three other male Gryffindors in their year, none of whom are particularly outstanding. At that point in the series, Neville was pretty much still a bumbling fool that couldn't do anything right. We don't see much of Seamus and Dean, and what we do see of them is certainly closer to lame than spectacular.
In addition, there was another factor that may have led Dumbledore to choose Ron. Recall what Ron saw in the Mirror of Erised:
Ron, though, was staring transfixed at his image.
"Look at me!" he said.
"Can you see all your family standing around you?"
"No – I'm alone – but I'm different – I look older – and I'm head boy!"
"I am – I'm wearing the badge like Bill used to – and I'm holding the house cup and the Quidditch cup – I'm Quidditch captain, too.
And Dumbledore's assessment of this:
"Yes and no," said Dumbledore quietly. "It shows us nothing more or less than the deepest, most desperate desire of our hearts. You, who have never known your family, see them standing around you. Ronald Weasley, who has always been overshadowed by his brothers, sees himself standing alone, the best of all of them.
Of all people, Dumbledore may have understood best what Ron really needed. Ron needed to be a prefect so as not to feel like a failure compared to his siblings. Even if Dumbledore did not have much to do with Ron throughout the series, he was privy to Ron's "deepest, most desperate desire", and he may have made him a prefect to help him start achieving it.
Remember, there may in fact be some precedent for Dumbledore choosing prefects based on "other" factors:
“What about you, Sirius?” Ginny asked, thumping
Hermione on the back.
Sirius, who was right beside Harry, let out his usual
“No one would have made me a prefect, I spent too
much time in detention with James. Lupin was the
good boy, he got the badge.”
“I think Dumbledore might have hoped that I would
be able to exercise some control over my best friends,”
said Lupin. “I need scarcely say that I failed dismally.”