Idk man, murmur seems pretty OK especially considering sr's are way easier to get maxed than mr/ur. She ends with like 30-60% resistance to every single attack type plus an extra 10% to all element resist. A decent hp Stat (about same as yshtola) and knows offensive spell and cura by default. I'm thinkin she could be worth maxing given the much lower cost to do so.
Unfortunately, SRs in this game have very low stats compared to MR and UR units. I recommend you to use MRs like Khury, Fina, and Rairyuu. at least for now. Even if you are a F2P, you can make MR units grow smoothly.
I would prefer if we had a tier list subsection for each individual Rarity as well as an overall tier list, I know some games some of the lower Rarity units can still do some work or can be just as strong as some of the higher units if maxed out, such as dragalia lost.