Pronunciation is always going to be tricky with an international audience. People will pronounce any name differently depending on their native language. Example: Hermione. I'm pretty sure most people didn't know how to pronounce that until the films hit... The books were still a huge hit. So I say, don't worry about that. (I will always pronounce "Shnee" like the German word "Schnee". Sorry. xD Too deeply ingrained)
Names will also mean different things in other languages or evoke different associations. Take the band Kiss for example - straightforward enough what it means in English, but in Swedish "kiss" is "pee". I bet a lot of people thought it was dumb at some point. They made it big in Sweden anyway. So don't worry about people thinking it's "dumb" either.
Now if you don't like the name and just want to shorten it to Rex, I see absolutely no problem with it. I think most readers know his nickname anyway. Most of us want to get rid of MS/HS luggage at some point :'D But if the only reason you want to change it is because of some (honestly really rude) "readers" calling it dumb or threatening not to read your comic because if it... Screw them. I mean honestly, if one name is enough to deter them from reading the comic, even if you fixed it, you might have, idk, one mouse too many on the next page for them and they'll go "this comic has too many mice in it I'm not gonna read it". Who are these nitpickers to say what you should do with your comic?
My personal opinion is that you should keep the name - Rexerse is unique, it's marketable, it's something you can google and find the comic with. Honestly! Google images spits out your character in the top results for me, and that is a real asset. And idk about my taste in names, but I like Rexerse.
Speaking of associations... When I hear Rex all I think about is my uncle's hyper-active fox terrier. A cutie with wayyyyy too much energy. And separation anxiety. Geebus. Howled all over the lake last time we left him.