In Japanese you can add -やすい or -にくい to a verb stem in order to say if something is easy or hard.
For example with 書きます if you remove the ending you have 書き.
書き＋やすい＝書きやすい(Easy to write)
書き＋にくい＝書きにくい(Hard to write)
食べます、食べ＋にくい＝食べにくい(Hard to eat)
With your sentence it is saying もっと書きにくいペン so it is asking for a pen that is "more easy to write (with)" or "EASIER to write (with) "