Is it ok to make kids write lines as a punishment?

Absolutely yes, but don't give them a specific time to write lines, or you are going to see only one line written after the 30 minutes is done. Give them a specific number of lines. And yes, this is a great way for them to learn handwriting, especially cursive, as that is disappearing more and more from schools. If my kids came home with a warning from the teacher, then they had to write her warning down as many times as I said to, and the number depended on how bad the warning was~ 20 lines for slightly bad, 50 for really bad. And there was no electronics for the rest of the night as it generally took them the evening to do the lines.

Probably - depending a little on the age of the child(ren). Another idea is for them to write a letter of apology, to include the reasons for their apology. To demonstrate why they upset somebody, for example. 

Good LucK!

Okay but when i write for too long my hand gets tired.  It’s an ineffective punishment.  Like when my school had detentions...why make us all sit in a room for half an hour doing nothing when they could be helping the student council with all the boring work we have to do?  It never made sense to me, i would’ve enlisted help from all of the detention kids to be productive at school instead

My Mother did that once, school did it a lot. You don't give it an amount of time because they could take 30 minutes to write one line if they want to, the idea is give them a number of lines and make the sentence to be copied quite long, that way they can't write deliberately slow, the time it takes is when they have completed the required amount. Give them a dunce hat too.

30 minutes is way too long. 10 minutes will do. As long as you aren't hurting them, that punishment is fine. 

It's better than the alternatives, that's for sure, I would call you that a "light" punishment.