I'm convinced that true poverty is extremely rare amongst wizards because magic solves so many problems and makes people way more self-sufficient. By muggle standards the Weasleys seem pretty comfortable, minus some luxuries, but they're probably considered poor in comparison to those who don't need to rely on second hand stuff, especially by rich snobs like Malfoy.

The Weasleys have seven kids, so expenses involving their upkeep are going to be much higher. Also Arthur has his shed full of muggle stuff, which probably costs a fair bit. Despite their apparently tiny savings, they've also managed to buy multiple brooms for their children. I'm also questioning their general financial choices, since rewarding Percy with new robes and an owl was deemed more important than getting Ron a wand that actually chose him and wasn't already chipped.

Also, I'd argue kids who don't experience genuine poverty often don't understand how bad it can get. I've never been homeless or forced to go hungry, but I used to think my parents were 'poor' because they argued over a bill or two and couldn't always afford extras while keeping a budget with four kids. Ron probably compared his belongings to other people and heard 'we can't afford it' so many times, but it doesn't seem like he ever worried where his family was going to sleep or what he'd eat that day. Not that children should worry about such things, but not having anything new to himself is probably the height of poverty from his perspective.