Hi, misdee. My ds2's thumbs did that. It didn't appear to affect the strength of his grip, which was vice-like, and his thumbs appear quite normal now. HOWEVER, he did develop trigger thumb in one thumb (where the top thumb joint ends up permanently bent forwards, as though about to flick a gun trigger) and had to have a minor operation to release his tendon from the tendon sheath. I'm sure this happening had some connection with his thumbs doing that, although nobody ever said why it happened - in fact, some people seem to think it can't happen in small children unless they are born like it, but I'm 100% certain he developed trigger thumb when he was over a year old.
ps ds2 is very hypermobile, but it doesn't appear to cause him any problems as there is nothing wrong with his muscle tone or strength. Ds1, whose thumbs didn't do that (he didn't have the grip strength to achieve it) is very hypermobile and does have problems, but not specifically with his thumbs!