We see two actual examples of squibs in the book:
Filch - he is not exactly a nice person, and (spoiler):
Arabella Figg - while she turns out to be much nicer than we see in the start, she's actually a bit weird and not the company that one will ask for in general.
We also have Neville who was treated like a squib for a bigger part of his childhood and it takes him years to recover his confidence.
The wizard community holds deep prejudices against squibs. Here what Muriel has to say about this:
“So you say, Elphias, but explain, then, why she never attended Hogwarts!” said Auntie Muriel. She turned back to Harry. “In our day, Squibs were often hushed up, though to take it to the extreme of actually imprisoning a little girl in the house and pretending she didn’t exist –”
(Muriel refers to the times two generations before Harry's and Ron's parents but such things don't go away for hundreds of years, especially in closed small communities like the wizards' one).
So being a squib means getting "hushed up" all the time - a terrible childhood. Behaving badly or becoming a very shy and cold person are the natural defences that a kid develops in such cases which often carries on as the kid grows up.
Being a squib does not always mean that one will become a not very pleasant person. It is just likely that it happens because of all the treatment that you get from the others.
Getting back to the original question - most probably on top of being quite a distant relative, the squib cousin is also not a very nice person. In such cases parents usually avoid any comments about him especially in front of an eleven year old kid.
