Would 蜜心 be a good Chinese name?

The name that you mentioned in the question, Mixin, or 蜜心 in Chinese, literally means honey heart. Since honey tastes sweet, this Chinese character could also be used as an adjective to mean sweet. Thus, 蜜心 could mean sweetheart. In my humble opinion, it’s not a bad name for a female. However, it could make people feel that this name sounds saccharine in a formal name of a person. So, I would suggest you change this name a little bit.

I mean, you could change the given name from 心 to 芯, 欣, or 馨. And these three Chinese characters share the same pronunciation with 心. So, three new names appear, i.e., 蜜芯, 蜜欣 and 蜜馨. Now, let’s look at the generational name in this name 蜜. This Chinese character is seen in the names of people, for instance, Sun Miwei, or 孙蜜唯 in Chinese, an actress from mainland China and Hu Midan, or 胡蜜丹 in Chinese, a singer from mainland China.

Here is a picture of Sun Miwei.

Here is a picture of Hu Midan.

Speaking of the Chinese character 芯, this could be used as a given name for females, for example, Milla An, or 安晨芯 in Chinese, an actress from Taiwan and Ma Guoxin, or 马国芯 in Chinese, an actress from mainland China.

Here is a picture of Milla An.

Here is a picture of Ma Guoxin.

Speaking of the Chinese character 欣, this could mean happy and pleased. And you could find this Chinese character in names of other people, such as, Jiang Xin, or 蒋欣 in Chinese, an actress from mainland China and Qiao Xin, or 乔欣 in Chinese, an actress from mainland China.

Here is a picture of Jiang Xin.

Here is a picture of Qiao Xin.

Speaking of the Chinese character 馨, this could mean fragrant. And there are people whose given name is 馨, for instance, Chao Yung-hsin, or 赵永馨 in Chinese, an actress from Taiwan and Zhou Zixin, or 周紫馨 in Chinese, an actress from mainland China.

Here is a picture of Chao Yung-hsin.

Here is a picture of Zhou Zixin.

After choosing a name among these three choices, you could add a Chinese surname before it, for instance, Wang, or 王 in Chinese, Zheng, or 郑 in Chinese or Zhang, or 张 in Chinese. Then this new name could sound more natural to ears of native speakers of the Chinese language.