I'm no video editing expert. I'm not an experienced photographer. I have never done any color grading of any kind, but I have watched YouTube videos on how it's done and am well aware of the power of those software tools: amazing.
Having said that, I've only used iMovie to edit my videos - I just taught myself and learned through trial and error. In my experience, iMovie does NOT make a video look washed out or bad at ALL. My videos look pretty darn good using iMovie and NO color grading. I typically watch them on a new 75" Sony XBR 4K HDR television in true 4K mode. They definitely don't look washed out, they look just as good as most of the DJI videos I see on line.
I'm not a good film editor, and I'm not a good photographer, and this definitely shows in my videos. But if your picture QUALITY is not good, I can promise you that it's not because of the iMovie program.
Of course iMovie won't allow you to do sophisticated color grading, so you can't make a BAD picture look GOOD, or a GOOD picture look GREAT, the way you can with Da Vinci or a beefier program like those.
I'm just saying, I wouldn't blame iMovie for making your picture quality look poor, because my videos' picture quality is pretty spectacular in most cases and I barely know what I'm doing. I film in Normal color mode in 4K 24FPS on the Mavic, manually adjusting the white balance and exposure depending on the shot.
just my 2 cents