I would check with the show organiser, the crossbreds are normally judged in the pens as well as being shown untrimmed/dressed (for lamb classes) at least for the shows we have taken crossbreds too. It is completely different to showing pedigree/purebred sheep which are normally shown in the ring, and depending on the breed trimmed and dressed etc.
If the shows you are taking your lamb to are judged in the pen you don't necessarily need to halter train, although it is as well to have them used to being handled. However last year at the last minute I got a call saying that I couldn't take my crossbred lamb as she was MV acc and all the crossbred classes were in the non MV pens together, then they said if I could halter train her (in a day!) she could come and be stood a little distance away from the pens for judging, got her halter trained however the judge didn't have a problem with coming over and judging her in her pen away from the others.
I wouldn't worry too much if she is a shearling and not yet trimmed, although best to get her done early if she is too plenty of fleece grown through, but it tends to be more commercial entrants in these classes so they are not necessarily prepared the same as pedigree sheep. As I remember it was a Suffolk cross ewe lamb that won the two shows we went to with crossbred classes so hopefully you will be in with a good chance with your ewe.