Is it incorrect to pronounce the name Ceri — which is of Welsh origin I think — as seri instead of keri?
It is pronounced KE-ri. You can find the source here.
Names are pronounced as the owner wishes them to be; imagine how you would feel if somebody told you you were pronouncing your name 'wrongly'. Ceri is of Welsh origin, and pronounced with a hard c in Welsh, so most Ceris use that pronunciation. But there is nothing to prevent a mother seeing the name in print and giving it to her daughter, pronounced with a soft c.
the name is of Welsh/Celtic origin and roughly translates to fair, blessed poetry. It is short for Ceridwen and is properly pronounced Keridwen.
That being said, my name is Ceridwyn, or Ceri for short. My mother liked the sound of a soft c as opposed to a hard c, so I've always been known as Seridwyn or Seri. Technically having "wyn" as opposed to "wen" is grammatically incorrect as well (WYN is masculine; WEN is feminine), but I quite like my name, as do others. People tend to like the sound of Seri; it sounds like a variant of Sara.
Note: Nobody is going to judge you if you like the name with a soft c ("s") [save for the Welsh], so say it however you want. Names should be fun!