Why does Lou scold Willy Mays Hayes after he makes a catch?

The proper way to catch a fly ball, which is taught from the very early leagues up through any instructional league, is to hold your glove up above your head, palm up, so your eyes have a perfect line of sight and you can "look the ball into your glove" (that is to say, watch the ball as it descends into your glove). It's proper form and the safest way to make a catch.

Willie Mays, whom "Wille Mays" Hayes gets his nickname from, was known to make riskier plays for the amusement of the fans. He was one of the first to adopt a "basket catch"; a type of catch where you hold your glove like a basket, below your waist. This required a bit more skill and ability to make some calculations quickly, as you can't line up the catch with your eyes and instead you have to rely on more of your judgement.

Lou Brown needs wins, and so wants the team to play using fundamentals and basic baseball procedures. There's no room for flashy plays or the errors that sometimes come with them. So he compliments Hayes on the catch, and then makes sure that he won't take risks in the future.

Why does Lou scold Hayes if he actually got the batter out? Is this some sort of bad catch to make in baseball?

Because it's better to be sure and safe rather than be 'clever' and risky.

Is it bad to make the catch? Obviously not...but it's unnecessarily flashy it if goes wrong he could miss or drop the ball and lose the out.

The point is to make the best and safest catch you can and guarantee the out.

He makes a theatrical catch that is not so safe. Keep in mind that the team is build with not-top-players (in some cases, even not-professional players), so every point they make is considered gold, so Lou Brown wants that the players make a safer (and so, sure) catch, without any risks