On one hand, in a design perspective it makes sense. Patrick's parents were initially made with the whole gag being about how much they eerily look like him because in that context the punchline was that Patrick is so stupid he doesn't recognize his own parents when they look just like him. When implementing them in an entire show having all the characters look the same is awkward, especially when one of the number one rule of good art direction is making sure each character has a distinct and recognizable silhouette. Add the fact they wanted to differentiate Patrick in his spin-off with a slight redesign with a shirt and the need to alter their appearance becomes understandable.
On the other hand though they look ugly as sin.
And this show was a stupid idea from the start.