I get this question in my DMs at least once a week. Here is my one-and-done reply.
No, you cannot install custom maps in Rocket League Sideswipe. There is no toolkit, no unpacker anyone shares publicly, and no legitimate mod scene. If someone is selling you one on Discord, they are either scamming you or about to get your account banned. Probably both.
'But I saw a YouTube video with a purple field!' That was almost certainly the Rocket Labs limited-time playlist that Psyonix runs sometimes, or a filter over a screen recording. Not a mod.
'But someone on Reddit said—' Someone on Reddit says a lot of things. Ignore this one.
Mods on mobile Rocket League would require rooting your phone, patching the APK, and giving up online play permanently. Nobody who is good at the game is doing this. Play the game as it is.