I have lots of experience with the Rosa v BowJ and Ness v BowJ matchups, but since nobody has put out anything about Rosa I figure I'll just stick to that.
Before I explain anything, it's important to note that in the competitive scene, and assuming customs on, you should always be using 1112 for this matchup, emphasis on the x1x2. Down 2 (impatient mechakoopa) is critical in this MU because while the the other 2 down B variants can be stolen and thrown by gravitational pull, it is impossible for her to throw back impatient mech - it instead explodes at her feet if she pulls it in.
As with many MUs against Rosa, the trick is to kill Luma before you do anything else. Surprisingly, unlike some characters, we Junior players actually have a good set of tools for getting rid of the pudgy little star, thanks to our clown kart dash.
Depending on how shield-happy your opponent is, you can very easily drive in and attack the shield with drift - because Luma isn't protected by shield, this can often take it out in one go. If you have an opening to do a koopa kart combo, I've found that side b -> jump -> bair on Luma works at any percent and does well to get some space between Rosa and Luma. In some cases you can also do clown kart to up b and drop the explosion between them.
So what happens if you're unable to kart your way in because you're being pestered by Rosa holding jab or spacing you out with single starbit shot (the preferred side custom for this MU)? This is often where the impatient mech comes in, as it can launch as far or further than Rosa's custom side B. You can even punish the jab-holding with a charged koopa cannon in some cases. Most of the time, however, I tend to approach with a clown kart in the air and approach Rosa (not Luma) with a dair, then focus Luma while she's hit.
The last resort for killing Luma, typically only good if you've knocked Rosa offstage, is side smash. This can consistently launch it offstage from close to center stage at most levels of charge, and if it doesn't it may outright kill it because of damage dealt.
Once you get rid of Luma, the MU is like any other, and Rosa alone has very few ways to get an advantage. Just land your ground techs when fair gimped, mix up your recovery options to avoid getting meteored (going low and just doing abandon ship vs going high, etc), and nail those kart combos while you have the chance
With the right execution, I feel that this MU is slightly in Junior's favor with a ratio of 55:45. I hope this helps anyone having difficulty with the matchup, and of course add on/critique what does or doesn't belong!