Premier League is very competitive. Newcastle management should have built a strong squad before the start of this season. Because they have a lot of money. But Newcastle management were not keen on strengthening their squad. As a result we see Newcastle's decline again this season.
They might have way more money after they got acquired by the Saudis, but they're still nowhere near the top premier league clubs. Depending on what criteria we take into consideration, by a quick google search Newcastle is somewhere between 7th and 11th place in the richest premier league clubs ranking. So if we assume that more money should convert to a higher league position, they're doing pretty much as expected.
Of course their poor performance in the last 5 games could be concerning, and probably won't improve on Sat 13th, when they're more likely than not to get beat up by Man City.