Is it easier for a native Mandarin speaker to write in Chinese compared to writing in English? What are the reasons for this?

Yes, writing Chinese is easier for them generally. On average, native Mandarin speakers speak better Mandarin than English. On average, they are more likely to have grown up and been educated in China than in a primarily English-speaking country.

English is quite a difficult language for anyone to learn to spell. It is also challenging for native Mandarin speakers to master writing in English. For these reasons and undoubtedly others, native Mandarin speakers will therefore, on average, write better and more easily in Chinese than English.