In May you will probably get some nice weather – if it is clear it will be nicely warm during the day, but at that time of year it can’t be guaranteed (it could just as easily be overcast or rainy.
I have never been to Greece in May, but I have been in late September and early October and it was easily sunbathing weather then.
FYI – Skiathos is a beatiful island but there is only one real ‘town’ there – Skiathos town. All the other built up areas are just collections of apartments beside the busy main road that runs through the (very small) island. I would only really recommend the main town (certainly at that time of the year when it won’t be very busy). Skiathos town itself is very cosmopolitan and has some lovely bars and restaurants though.