Copied from a reddit reply:
'Jaskier' is an original Polish nickname of Dandelion (real name Julian Alfred Pankratz vicecount de Lettenhove). In Polish the word 'jaskier' refers to a yellow flower of genus Ranunculus* that bears no special connotation other than being a nice flower. The most common English name for that plant is 'buttercup' that, incidentally is a common term of endearment used in reference to kids, especially little girls. Thus, to avoid association with infantility (although Sigismund Dijkstra would have something say in that matter), effeminacy or condescension, translator have chosen other flower name that bears no special connotation. Dandelion, being also a bright yellow flower was an obvious choice.
If there is a word 'Jaskier' left in English translation then it must have been an error of the publisher. 'Jaskier' and 'Dandelion' are and always has been the same person.
*On a side note, in German and Swedish translation the bard is known as Rittersporn/Riddarsporne, after another plant of genus Ranunculus known in English as 'larkspur'.