I haven't ever done what you're doing, but I'd guess there's two problems at play here.
Problem 1: The Wii U (and Wii, and also the N64 actually) is a big-endian console, so N64 saves are written and read "backward" relative to how they're handled on little-endian platforms, like any modern PC or the Switch.
Solution 1: ED64-Saveswap will allow you to byte- and word-swap your save to arrange it in the direction little-endian consoles expect to read their saves. This is a GUI tool, so you shouldn't have any trouble using it.
Problem 2: RetroArch Mupen64+ has its own custom save format which is distinct from any other emulator, so there's no kind of save file you can just rename and start using on that RetroArch core.
Solution 2: ra_mp64_srm_convert is a tool which converts between traditional N64 save formats and RetroArch Mupen64+'s dedicated save format. It's a command-line application, but if you don't feel confident using the command prompt, it is designed to "guess" what you want based on the file you supply if you just drag and drop your save file onto the app.
Between both of these tools, I think you can probably get your save working on Switch.