The way the vast majority of people play here is to give Undo automatically in situations where the player got no advantage. Especially true when it was a mis-click. Especially especially true for online situations that simply would never happen in real life. (In my real life games, people show all their treasure cards and everyone knows that Bank is played last [or maybe right before Fortune, in an edge case]. To give just one of a thousand examples).
So, since practically the entire Dominion online community plays with the standard, OF COURSE you come across as some version of jerk or rude or poor sport, if you post about wanting to change the gameplay, to accommodate your values at the expense of 90+% of other players. Your post, and subsequent responses, suggest that you are NOT, in fact, a jerk. You made a reasonable and thoughtful post, you got honest feedback, and then the thread has, slowly, become a bit more personal and hostile.
I'll observe that an extremely kind and gentle and wonderful person can still come across as a jerk, even though that appearance is unfair and unjustified. One way to disabuse others of an incorrect first impression might be to make a conscious effort to not add insults, to not ratchet-up the personal comments. This can be difficult when one feels that she or he has been unfairly attacked. Just my 2 cents here.
I'll note that the Chat function can be very helpful, to address your concerns about how to handle Undo requests. I'll offer a suggestion to you: At the start of a game: write in Chat something like, "Hi. Good luck. You should know that I prefer to not grant Undo's, and I'll never request one myself. Is this okay? If not, please play extra-slowly, and I will not be hurt if you want to stop after only one game."
Or, "Hi. I wanted to mention upfront that I prefer to not grant Undo requests. If you do end up asking for an Undo, please explain why here in Chat. Good luck...I'm looking forward to playing with you."
I think there are ways of telling people, before a game even starts, how you feel, and ways of doing it nicely. You will certainly end up with fewer people wanting to play with you. But, probably, your games will be more enjoyable for you, as you'll get far fewer requests coming at you during games.
(p.s. Have you really been playing games where people are making Undo requests many many times during the game? That would be incredibly annoying for me as well...I think a few people made 2 such requests in a single game, and that is the most I've had to deal with.)