In ride queues, I have never experienced queue jumpers at DLRP.
Character meets, however, are a whole different matter. I have heard here that it comes as a shock to lots of people who have previously been to WDW, when they experience the character scrums at DLRP. And they can be just that - scrums, free-for-alls. Some adults think nothing of barging little children out of the way so they can grab the character for a picture or autograph. CMs with the characters are getting better at dealing with this, but they have a long way to go yet.
If you don't want to get involved in character free-for-alls, consider the queued meet n greets, which take place in both the main park and the Studios, and which are advertised in the programmes (character name, location and times). Or, consider a character meal, where the characters come to you.