Absolutely, give them a good haircut right away. You should also be fertilizing them frequently with a low dose of soluble fertilizer. Some do it at every watering, but I don't feed my outdoor flowering containers that often.

It sounds like they might be in a location that received full sun all day long. Is there a place you can hang them where they might not get hit by the strongest afternoon sun? It's SO hot where you are right now!

Totally different zone up here, but just to chime in - I shear my petunias back mid-season (usually around middle of July). They look cr*ppy for a week or two afterwards but fill back fairly quickly and then the flower display picks up again. If I don't shear back after they poop out mid-summer, well, they're just pooped out then...

Next year, if you buy babies again, pinch (tip prune) them right away. It's hard to do, remove the pretty flowers you just bought, but this usually encourages lateral branching and fuller plants with many more blooms on each.