When streaking microbes on a plate to test growth after uv exposure, does how I streak make a difference?
I work with e. coli myself though I do not know if your special strain has anything drastically different than my own so I'll just give advice that I've gotten from post docs and other PhDs on how to handle cultures.

(Note: This is methodology that I can't really find a "source" for so take with a grain of salt and change for your needs)

Typically when streaking I try to ensure that I make the streaks light enough so there aren't colonies within vicinity of each other. It's hard to really give a quantitative answer on how to do this so you will have to experiment on your own so your streaks aren't laden with massive amounts of colonies next to one another. It should look like consecutive "beads" on a string. This is generally what I try to accomplish with my colonies. Keep in mind I am focused on cloning plasmids so I'm not really concerned about experimenting on the e. coli themselves, just getting individual populations that do not have mixed variations of my plasmid. This is why having colonies with good separation is important for my particular project.

As for making sure they are all being affected by the UVC, I'm not entirely sure if having them to heavily grouped together would "shield" from UV. I feel as though UV can easily penetrate through e. coli colonies but I guess its not out of the question. Best bet is to keep your colonies well spread out, enough so you can pickup a single colony with a pipette tip and nothing else. This will make any genetic experiments you may want to do a lot easier as well. Which I think would be interesting if you are interested in the mutations occurring.

Is there something about your project that requires you to streak these out on agar plates? I saw comments suggest liquid broth. Maybe that's easier to do to make sure everything is exposed and then streak them out on a plate to see growth and/or test for mutations.