I think Goku is a really honest character most of the time, but it’s interesting to think about the few moments where he might have lied on purpose. He usually tells the truth but sometimes he might say something tricky during a fight to protect others or surprise his enemies
There was one instance in the Oozaru Baby fight where SSJ4 Goku was practically spent, energy-wise, but wanted Baby to think otherwise.
What you mainly get from Goku is faith in himself to overcome large gaps in power. I wouldn't put this in the same category as lying, generally speaking (outside of the above example, which would be perceived as brave rather than malicious from the heroes perspective anyway).
Btw he does tell some minor lies when it's convenient. Like when he said to Piccolo he likely couldn't have beaten Fat Boo. Piccolo pressured him tho and he kinda half admitted there that he could of but he wanted the boys to do it as they could with fusion and so they can be responsible to protect the Earth as it's not his place in the sense he was dead, so he wanted them to do it as a way to give them experience and a sense of what it means to have such a role. So he lied but he had his reasons.
When he lies the reader knows it's a minor stuff and not with malicious intent.
He lied about getting elder Kaioshin a meeting with Bulma too, as he realized he was a pervert and wanted to see his special power to make them stronger.
Tbf I don't think Goku would have beat Fat Boo anyway due to the latter's regeneration, plus SSJ3 being quite taxing even with a dead body. Keep in mind the form drained all the remaining to Goku had left alive.
^ Refreshing to see a viewpoint that gives Buu credit in that scenario. While Goku has a win-con against Fat Buu, his energy drain casts a cloud over him that requires precision to get the job done.