I fight Beaver pretty much every week and I'll agree, I think it's in Lambda's favor. Closing in on her is hard. Best way to do it is jump in + barrier. Mu has to be really cautious. Lambda's 4D is something you really gotta watch out for. Get hit by that in the air and she'll 236C into big damage. Mu's lasers might go full screen, but they don't have a hitbox on activation like Lambda's swords. I'd say this matchup is difficult for the same reason Haz is, in the fact that it's risky to set stuff up.
However if you can pin her in the corner you can win, it's just catching her that's hard. You have to be really patient and close in slowly. Only set up one stein at a time between her zoning phases. Use Habaya cause it'll cancel her swords and buy you some time.