Is restarting a level good in video games?

Depends on the game. Some might punish you in score or lives for resetting repeatedly, some don't at all. And anyway thats the definition of practice, doing something over again repeatedly until you get it perfect, and practice is good. 

Sure, why not? It might help to know what game you're playing and what level you want to restart. But a lot of video games are designed to have individual levels replayed, or you can load a save and replay from an earlier point, anytime (assuming that you've been making save files along the way). If you're not sure, make a save after you finish the level, then reload an earlier save and try again. If you prefer the way you played the first time, then quit and reload that save. Don't use the quicksave, make hard saves in separate save slots. 

just try it and see and then you'll know