Although you are likely to get a better sound from a better ukulele and the tone of any ukulele will be improved with decent strings that are appropriate for the instrument, these are not the main factors. The real answer is PRACTICE.
If you gave an absolute beginner a violin to play, they would sound just as bad on a cheap instrument as a $1000000 Stradivarius. If you give a cheap instrument to a real expert, you would be hard-pressed to realise that it was not a high-quality instrument that they were playing.
Take Aldrine, for instance. Why does he sound so good? Is it that nice Kamaka that he plays? No, in the end that is just an ordinary workshop-made ukulele like thousands of others. He sounds so good because he plays every note carefully. Notice the left-hand vibrato, the careful placement of the fingers. His right hand in just the right position, his fingers at the correct angle to get the tone he wants. Nails smooth and carefully sharpened. When he strums there is an evenness of dynamics. He is rhythmically precise. I could go on. I pick Aldrine because everyone knows his playing. He make the simplest thing sound good. Because of the sound he gets, he is actually a much better player than many people would believe.
I think we need to stop thinking of the ukulele as an easy instrument, one that anybody can just pick up and play. It is not about buying a better instrument and becoming a better player. Yes, having a good instrument helps. It gives you confidence and makes you feel like you are making progress. A good instrument may play in tune and this is important. But you can easily make a good instrument sound out of tune just by pressing on the strings to hard or by not getting it in tune in the first place.
I feel quite strongly about this. And I mean no criticism of anybody. But I believe that the ukulele is a serious musical instrument. Some of the best players on YT use inexpensive factory-made instruments and make them sound great. Why? Because they are passionate about their playing.
Sorry to go on so long.
KEN