you should never feel intimidated by them, and please do message them!
Big youtubers might not have the time to reply, but I'm sure many of them do read through as many comments as they can, and your words can still get through to them.
I'm not a big youtuber by all means, but even at this small level I've had people tell me they were too scared to talk to me, or ask me to play a game with them, or ask for feedback, and a lot of the times people are in shock when i reply to them, or even when I'm just commenting on random videos on youtube like why is it so bizarre for a youtuber to comment on vids they like? you always get a "what are you doing here" when the answer is obviously "same as you" lol
on one hand it is very flattering, but on the other hand it's also quite sad, because people do get put on a pedestal over a number. and it does discourage people like yourself from reaching out, and replying to their comments.
Getting into twitter has been great because it encourages everyone to talk direct, and noone seems as shy there
TLDR; - respond! don't be shy and go for it
