Er, no? An emoji reaction for “+1” is available, and I guess you could mangle that into meaning +1 or “thank you” in a lot of cases. However, it’s reasonable to assume that if people just want to give a “thumbs up” reaction they will, since it’s less effort than actually writing a post to reply “+1”.

There are of course lots of short replies that simply aren’t covered in the odd 9 reactions that we are allowed to use - “-1” being the most obvious missing one.