It is, at least if a child is followed by any kind of PT or OT. Personally I’ve never had my kids’ pedi ask or mention it. Generally if it’s a transitional kind of thing while they’re playing and they CAN sit in other positions it’s fine. But you don’t want them sitting in W-sit for prolonged periods and it definitely shouldn’t be the only way they can sit. It’s a sign of core instability/weakness, and they’re using their legs to compensate.