There is plenty of online documentation but it won't tell you, I believe, whether or not it is illegal. That is because there will not turn out to be a law saying you can't put change into other people's meters for them. Instead, you possibly could be charged with tampering with a meter (there is an ordinance against that) or perhaps interfering with an officer in performance of her duties. Then, it would be up to the judge to decide whether what you were doing amounted to that. Case law, if any, for minor ordinances is generally not accessable because it usually doesn't get compiled anywhere.
Since you specify Los Angeles, I can give you some online sources. Also, you can even email them to ask the question at:
LADOT.CardandCoin@lacity.org
Or you can pore over the entire ordinance book on-line looking for anything that they might throw at you.
But it all looks pretty moot to me given the new high-tech meters they have begun installing. You don't just drop in change any more, so read all about it on their site. It looks like it would be pretty expensive to try too.

Reframe the question, and you will have your answer:

"Is it illegal to give money to a homeless person?"

"Is it illegal to drop a $20 bill on the ground?"

(The metermaid in the Jackass episode is not a lawyer, nor even a trained officer of the law.)

To be honest, I've never heard of it being illegal but you never know. Personally, I always thought it would make my day if someone topped up my metre and I think it's thoughtful but well, apparently, cops have a different opinion...:P

To find out you could call your city police, look it up perhaps or call your city hall. But be careful about the internet one,because some people might not necessarlily have the right details.