Are they a "known" senior skater? If so, I would think a poster is okay. Most skaters really appreciate their fans. Just be polite with the poster, and don't block the views of people around you.
But if they are a local skater who is still "unknown", more of a hometown hero, it might make them a little uncomfortable. I would still say (if their body language suggests it is an okay time, as the other poster mentioned) that even an unknown skater would be happy to sign autographs (maybe even more excited than someone who has to do it all the time.)
(On a random note, my husband and his buddies brought a "D" and a fence cuttout sign to a synchro competition once. Everytime we skated near that side of the ice they held them up and cheered. After the competition he played dumb and said "What do you mean there is no defense in synchro? That sign works for every other team sport!" I thought it was hysterical, and loved knowing I had a cheering section. Not the same thing as a senior skater, but I think everyone likes to feel their skating is appreciated! I'd leave the signs to entering and exiting the ice though.)