I can help you with the Mu match-up a little.
Ground is the best way to fight and approach Mu. You will not win an air-to-air battle with Mu, don't even try. Avoid being in the air, and play footsies. Mu's 5C is way slower than Ragna's 5B, and it barely outranges his 5C. A lot of Mu's 5C attempts can be stuffed by either of those. Ragna's j.C is pretty decent as a jump-in now, since Mu's anti-airs got nerfed, but do not abuse it too much. When you do get in on her, watch out for her DP, it'll eat all attacks and throws, and toss you to the other side of the screen and reset all of your hard work getting in. Your best bet is to bait it, and punish accordingly. Don't take too many guesses against Mu, her CH damage reward is too high, and she can easily steal momentum from you to set up her oki.
I don't really know too much about Lambda, except that she's much worse up close than Mu, and she has no defensive options other than counter assault and reversal super, so have at her. Against Lambda's swords, super jump forward and block. You will slowly make your way to her.
Basically, both match-ups are all about playing lame, and keeping him out. Remember that you're Ragna, and you have "I got hit by a truck" damage, so they're going to make you work to get in.