I think it would be rude for chopstick users to laugh at fork users, but I don't think it's rude to use (or attempt) either utensil, no matter what anyone else is using.
I am not very good with chopsticks. I can usually eat sushi with them, although I have problems if the pieces break sometimes. At a few restaurants, I've been given a fork (for "just in case"), but usually not. None of the Asians I've eaten with have really commented on my chopstick etiquette, although they have tried offering me pointers to make it easier.
I haven't yet attempted other things, like my favorite honey chicken on rice. I think I might drop it all over the place. I really should get some connected chopsticks so I can practice at home where I won't embarass myself.