is the Ponziani Opening good for beginners? (as in 600-750 elo)


It's a solid setup at any level. And because you have a decent position right from the start, it's a more surefire way to reach an endgame. That said, the tactics and traps it keeps are pretty advanced. So it's best to study all of the known theory before playing (do homework).

No, it isn't really good at any level, considering that it's competing with the Scotch, Italian, and Spanish.

Absolutely!
Here are some of the 'easier' (i.e., less 'theory' – documented lines and variations - to learn) 1.e4 openings for White which are sound and very playable at the amateur, club level. Typically, the 1.e4 games are so-called "Open" or "Semi-Open" games, where lines (files and diagonals) can more readily be opened which tend to lead to games of a tactical character, as opposed to "closed" openings characterized more often than not by blocked centers which tend to result in a more maneuvering, "positional" style of chess early on in the game...
Scotch Game
https://www.youtube.com/results?search_query=scotch+game
https://en.wikipedia.org/wiki/Scotch_Game
Ponziani Opening
https://www.youtube.com/results?search_query=ponziani+opening
https://en.wikipedia.org/wiki/Ponziani_Opening
Vienna Game & Gambit
https://www.youtube.com/results?search_query=vienna+game
https://en.wikipedia.org/wiki/Vienna_Game
Introduction To The Vienna Game & Gambit
https://www.chess.com/blog/RussBell/introduction-to-the-vienna-game-gambit
The Italian Game – more challenging to learn (i.e., more 'theory') than the above, but a very rich and sound opening complex, which can be played for the rest of your chess career and at the highest levels...
Introduction to The Italian Game & Evans Gambit
https://www.chess.com/blog/RussBell/introduction-to-the-italian-game
Bishop's Opening – this is not an opening per se, but an opening 'gateway', leading to a rich variety of openings (useful for keeping your opponents guessing!) including several of the above, especially the Italian Game...
https://www.youtube.com/results?search_query=bishop%27s+opening
https://en.wikipedia.org/wiki/Bishop%27s_Opening
If you prefer to play 1.d4 as White, then perhaps the 'easiest', yet effective, openings to learn are...
The Stonewall Attack - I consider this to be a precursor to the London System (there are some similarities, and the Stonewall is easier to learn), and very playable at the amateur, club level...
The Stonewall Attack
https://www.chess.com/blog/RussBell/stonewall-attack
Introduction to the London System & Jobava London System
https://www.chess.com/blog/RussBell/the-london-system
Chess Openings Resources for Beginners and Beyond…
https://www.chess.com/blog/RussBell/openings-resources-for-beginners-and-beyond
For more good chess resources, be sure to check out my blog...
https://www.chess.com/blog/RussBell

It's a solid setup at any level. And because you have a decent position right from the start, it's a more surefire way to reach an endgame. That said, the tactics and traps it keeps are pretty advanced. So it's best to study all of the known theory before playing (do homework).
The thing is, everything you just said applies to the Scotch, which is a much better opening overall. The Ponziani is to the Scotch what the Alapin is to the Open Sicilian. Both let Black equalize with 3... d5, both score worse, both have less theory because they're worse moves, both are playable if you understand all the theory. I wouldn't recommend them to beginners.