We see two actual examples of squibs in the book:
Filch - he is not exactly a nice person.
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.
Although it is a bit of a speculation, I guess being a squib means having a terrible childhood. It takes a strong character to overcome such a thing and accept your own limitations.
So 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 years old kid.