Your perception is indeed wrong. At least with respect to the sites you mentioned:
Site Total Qs Downvoted Qs Percentage
Ask Ubuntu 238559 13251 5.55
Unix & Linux 118261 9744 8.24
Stack Overflow 14258884 1676744 11.76
The table above shows1 the total number of questions asked in the past month (since 20-10-2017), the number of those that have been downvoted and what percentage of the total questions have been downvoted. As you can see, this site is actually the one with the lowest percentage of downvoted questions, not the highest as you are suggesting.
And these are the statistics of your own questions (all the questions you've asked on these sites):
Site Total Qs Downvoted Qs Percentage
Ask Ubuntu 149 12 8.05
Unix & Linux 66 5 7.58
Stack Overflow 344 23 6.69
So yes, you do seem to have a slightly higher percentage of downvoted questions on AU, but the difference is negligible. At least with respect to U&L, anyway. But even when comparing to SO, the difference is 0.013, which can't really be called "large".
So, in conclusion, I'm afraid the numbers don't support your perception that you're being downvoted significantly more here than on other sites.
1 I used these two data.se queries to get the numbers: Total Questions, Downvoted Questions.