According to Red Rocket director and co-writer, Sean Baker, it's up to the film viewer's interpretation, and he also added that the parts of the film where the color red was applied are "very interesting". From a Q&A session at Viennale 2021 (quotes below paraphrased slightly):
Question asker: Why is the film called "Red Rocket"? Is that like a sexual word*? Or...
* Note: It's unclear to me if the question asker said "word" or "way".
Sean Baker: The very simple answer is that there is an English slang term. And I may as well just tell you what it means. It's a little crude. It's a dog's erect penis. […]
But also it's up for interpretation. Like some people think she's red rocket, some people think the truck is red rocket. Some people, you know… My sister who is the […] production designer on the film, she was playing with red and blue a lot, and shades of red and blue but where she would apply red is very interesting. Yes, so… But again, up for interpretation.
Video of the Q&A session (the question was asked at 19:33):
There are many parts of the film where the color red was applied. The most prominent of which is on Strawberry. Strawberry is red-haired, was wearing red the first time Mikey met her, and drives a red truck (the "truck" Baker was referring to).
My personal interpretation: the "red rocket" is Strawberry. Strawberry has that red rocket effect on Mikey — she's something that stands out in Texas City (to Mikey). (Mikey: You're like an extraterrestrial around here.) Mikey was smitten when he first saw her and his obsessive desire to turn her into an adult film star drives his actions throughout the rest of the film.