If you’ve carefully tuned your uke (and its setup and intonation is at least OK) and your chords still sound a bit sour, you may be pressing too hard. You need just enough pressure so that all the fingered strings are firmly pressed against their frets. No need to squeeze ’em to death.
This can be an especially tricky thing if you’re coming to uke from a guitar background like I did. Your instincts are still set up for 6 steel strings instead of 4 nylon ones.
You might also grab some chords extra hard because you’re trying to go at speed through a tricky part of a tune and it makes you nervous. (see also: Slow down when practicing)
If you squeeze too hard you can end up stretching the string(s) sharp, often because you’re pushing or pulling them sideways on the fretboard. It also tires your fretting hand and arm more quickly.