AS u/buckeyechik suggested, carrying ponchos gets annoying. Personally I just embrace it. You're going to get wet on the Jurassic Park River Adventure (maybe) and definitely on the Pluto's Barge...and being wet will feel good with the "Feels like 103" degree weather. So wear things that will quick dry...wicking t-shirts. Swimsuits. For shoes I now only wear these water shoes - they're comfortable enough like sneakers for all day but water goes right through them like sandals.
​
For my day, I plan on getting wet. If it's not the rain, it'll be the water rides. The only downside to the rain is that the fast coasters can make rain drops feel like hail. So I plan for the indoor stuff in the afternoons. Aim for the Bourne Stunt show and Horror Make Up show between 2 and 5 along with the other indoor rides. Do all the outdoor stuff before and after.
As everyone mentioned - it's definitely still worth going. Most of the time the rain is only for an hour or so, definitely check the hourly weather that morning so you can plan the day better. Most of the outdoor rides shut down for lightning but will continue to run if it's just rain. Although in my opinion that's still a bad option, I did Hagrid's in the rain and it's not a great experience. The rain can feel like hail at high speed.
Keep in mind that Universal has a lot of indoor options as well, and the Islands of Adventure park has water rides. So come planning to get wet whether it's because of the rain or the Jurassic Park River Adventure or both at the same time. Hit the indoor rides during the actual rain, and appreciate how much emptier the park is after the rain because many people leave.
Ponchos are popular with the tourists but I don't get it. It's way too hot to be hiding under plastic insulation. I personally bring a water proof fanny pack which is also allowed on many (not all) rides so you can avoid lockers. When it rains or I go on the water rides, I take my socks off and usually keep them off the rest of the day unless my shoes get dry enough to put them back on. But I also wear water-sneakers so the shoes are really a non-issue. These things have changed my theme park life experience dramatically! They're comfortable enough for the miles of walking and perfect for the rain or water environment and dry quick so I can have my socks back in no time.