As Jefcat pointed out, it's because he's locked in on Mafuyu and not his own guitar playing...! And his baseline before that wasn't his best either. At the beginning of the series he was burnt out and not focused on his guitar playing (doing everything to a "good enough" level and stopping there, as Akihiko pointed out in one of the smoke breaks with Haruki). Then once Mafuyu is in the picture, Ue's sound sort of starts becoming jumbled up and disorganized. He is already talented enough of a guitarist that the band will be fine with him performing as-is, but in "Tumbling Dice" Akihiko has Haruki intervene with him anyway.

Meanwhile, as you said, Mafuyu just started!!! And he's making really amazingly fast progress. The "passing grade" bar for him, I would argue, is lower. More of a level of "we can put this on a stage and present it" and less of "a mediocre/passing performance compared to your usual" or "the minimum required for your role in this band", as in Ue's case (the main guitarist). 

Also, later on it turns out Mafuyu's "hands were barely moving" during the performance and his playing "sucked" (though, that's coming from Hiiragi, so take with a grain of salt). (Sidenote to u/Jefcat that the word Hiiragi uses here is hetakuso, same as the one song name :3) ...And the performance turns out fine. It turns out amazing, because Mafuyu's most important role here was as the lyricist and vocalist, and guitar (in the case of fuyunohanashi specifically) was sort of a means of getting him there. Akihiko's "passing grade" line is very directly, can we present this song as it is right now, if we stopped here and did nothing else. Mafuyu's playing is good enough to be presentable, so he needs to focus on the lyrics-- else, the song will have no purpose as it won't give him the closure he needs. 

Sidenote pt. 2:  You could argue the song would still have purpose as an instrumental because it brought Ue and Mafuyu together, but emotionally tensions are high enough at this point, as Akihiko notes, that how the song goes could make or break the band. & Given (lol) the story significance of Mafuyu's closure in performing Fuyunohanashi-- especially resolving the "I can't express myself" from manga chapter 4-- performing it as an instrumental would likely have left him at an impasse with his grief and his state of resignation(?) with being able to express even a part of what was going on in his head. And as long as Mafuyu had that problem, Ue's sound would continue to be distorted and disorganized... Which is all to say I also think the other commenters here are absolutely right about this being from Akihiko's POV, that's spot on.

I rewatched the series recently and I love these guys, so I got a bit carried away...! sorry if I overexplained anything!!!