Personally I have found core strength work and pilates helpful. Not because it makes you a better cyclist but it means you can sit on the bike for longer with less discomfort around back/arms/neck.
More helpful than spending the same time riding your bicycle instead? I have my doubts!
I would, tentatively, say yes, at least when the aim is to reduce discomfort. Not sure if it is worth doing more than 3 hours/workouts a week though, if that means less riding. Pilates and yoga classes tend to have a fair amount of stretching included, which may also help.
There's many different forms of yoga, coming to an Ashtanga class when you're used to doing yin-yoga will be a bit of a shock I think
(the reverse may also hold).
Power yoga may be a good place to start, tends to be more strength-oriented and less philosophical than most other forms; though the teacher also has a big effect on this. I have found the teacher makes more of a difference with yoga than with Pilates. If you do a Pilates class and don't like it then find something else; if you do a yoga class and you're rolling your eyes, it may still be worthwhile to try a different class and/or style with a different teacher.