I'm assuming you have the free Barbara for making AR 20? I think she's worth using since she's one of the only healers available for F2P without going hard after a 5*. I thought she was good enough that I spent all my spare primogems on Venti banner, not in the hopes of getting Venti, but hopefully get a couple extra constellations on her and Fischl.
As a healer or support though, you don't need to go HAM on making sure she is max level. You should typically max level 1-2 of your primary DPS, then later work on your supports and healers. So for right now, my party is Chong/Fischl/Xiangling/Barbara at 60/60/50/50. I also got by for a long time with Barbara at 40 in that setup as well.
Also, highly recommend giving Barbara the Thrilling Tales of Dragon Slayers. It's a 3* book, so easy to multiple copies to max refine it. It has +% HP as a primary which helps her heals, and it's pretty easy to swap to her, pop her HoT, then swap back for the damage buff. Not much of which really depends on her level. Plus all the HP makes her fairly tanky in spite of being underleveled.
Having a dedicated healer is overall pretty good, but the one place it didn't necessarily help though was those first floors of Spiral Abyss since your main opponent is the clock. I had the hardest time on 3-3 but managed to beat it using Anemo traveler's charged attack and Kaeya's final combo hits to knock guys off the platform, so it was more about technique than anything else.
