Well, it looks like you certainly got some responses. Renee, you may want to explain a little more of what you are asking and the situation. I know Shetlands well and although they are one of the strongest “ponies” for their size ratio, it is sometimes very hard to stay with them while in the saddle. Yes, your legs do hang down and the weight is a bit much for any kind of consistent work or even over a few weeks/months however “light” work it might be..
I am English and have competed all over Europe and now in the US, so to answer your question: In my opinion from what I have seen, ponies (big or small) would be frowned upon if an adult were riding it. Ponies are becoming more popular in the US but they are still not totally accepted yet, I do hope it will get there.
There are a lot of variables here which have not been explained. Is the Shetland a stocky sturdy one, that could handle more weight for some schooling or more of a fine Shetland who probably will not be able to handle the weight. Is the rider experienced or not, if the rider were more experienced than they would probably be able to distribute their weight more evenly and not have it all on the saddle on the spine which is where a novice tends to carry their weight while learning. Is the rider learning and the pony more experienced so that the pony can teach the rider or is it the other way around. Does the tack fit correctly, is there a sufficient nummah (saddle pad). I have seen, much too often here that tack does not fit correctly and then the horse is hurt somehow or can not move to it’s full potential. There is too much of “fit the horse to the tack” not “fit the tack to the horse”. It really comes down to the situation but if you have done your training and exams in the UK, you should have a good base and horse care/knowledge.