I read some praises towards Rikako, especially in the song with Moe-Muro and some praises were kinda over the top especially compared to Moe/Muro. I do think she has improved her singing in a decent kind of way for one of the worst singers of the group. Like, she doesn't sound off tune or strains much but she isn't good either for her to get more than she gets or ignored the best singers for her to get that line.
Ikuta to me has shown the most improvement regarding the bad singers (like her line in I'm Lucky Girl sounded great compared to Fukuchan imo) enough for her to not to be that girl that sounds awful that stands out for that in particular, which is why I think people react better to how she sings at least one liners than with other girls. Like, she is in a group that has some good moments but generally you won't trust them with hard lines and even with simple lines they can sound weird (Maria, Ayumi). Before she was less than the average singers and of the two worse. I wouldn't say she is good but I think she won't butcher a one liner.
I don't see how Ogata could be worse than Ikuta currently, at all.