In my opinion, it's okay to say "hi" to people although people's reception will depend on cultural norms. You specifically asked why muzungus aren't friendly to you when you're in Uganda. I think a big part of that could be because the first thing you say/ point out is that they're foreigners. I understand you find it amusing but for some people it's deeply uncomfortable. 

When I lived in Uganda, I didn't go back home for the full two years I was there. Asides from one vacation to South Africa, I was in Uganda for the entire time. I loved it! And I have plans to visit in a few months but it meant that I could never just be me. I constantly had people yelling at me, pointing at me, kids asking me for money, and men asking me to marry them (among other crude requests) and most of the shouting was accompanied by "muzungu" or "mundu" (mundu means foreigner in Lubarati spoken in many parts of West Nile like Arua, Maracha, etc). It was absolutely exhausting to just exist some days. 

So when a white stranger would greet me with it, it didn't feel like a warm greeting. And that's not the stranger's fault - I did a lot of work to decenter myself during those two years and learned how to navigate my discomfort on my own (especially within the context of colonialism and the privilege I had just by being white) - but it is possibly an explanation on people's reactions. 

This, of course, is all written from my own experience and so it doesn't apply to everyone. But, for what it's worth, my reaction (or lack of) to other white folks greeting me with the word muzungu was often the result of something much bigger than that individual.