Kenta is always a series of decisions because while you can literally translate his words, you have to do a bit of juggling to convey the way they come across.

What he literally says is:
能書きばっか締め直せよふんどし

Which you could literally translate as "all you do is talk yourself up; tighten up your fundoshi back up"

the important word that makes this hard to directly translate while keeping the meaning clear to an english speaker is fundoshi, which is traditional Japanese underwear, and specifically the idiomatic phrase ふんどしを締めてかかる -- literally "[tie/fasten] one's fundoshi", meaning to stop messing around and get to serious work. I read this, coming from Kenta, as being deliberately a bit crass.

So in order to kind of get across that level of childish provocation, that's why I translate it in this context as 'nut the fuck up', lol.
The problem is that this then kind of makes "all you do is talk yourself up" kind of feel like a weird precedent (it sounds fine in Japanese, I just think a direct translation kind of parses weirdly in English) so I scooted it around to refocus the translation of the line being about running your mouth instead of doing the work, especially in the context of being @ cozmez. Coming back to it a week later I do feel like I could probably find a better phrasing, but there was a logic to my twiddling, lol.
(But the "nut up" is staying, i stand by that one ww)