I can't say that this answer must be right cuz I didn't confirm it. But I would like to tell you what I think about it. So let's see these 2 examples. (Actually they have the same meaning) 1. 我的汉字写得很马虎。 2. 我的汉字写得马马虎虎。 These 2 sentences are both right and they share the same meaning. But why the first sentence has 很 but the second one hasn't it? What I think is: 很 in the sentence means VERY and it emphasizes that your Chinese characters writing is VERY not so good, which is worse than NOT GOOD. So here 很 is to deepen the degree of the adjective 马虎。I think in Chinese, for some adjective, we also have another way to deepen the degree of them. Like in English to transform BAD into VERY BAD. Let's use 马虎 as an exmaple. 马虎： not so good 很马虎：very not so good Here we can see that 很 is for deepening the degree of 马虎. Actually there is another way to deepen the degree of 马虎, which is using 马马虎虎 instead of 马虎。 In other word, 马马虎虎 also mean VERY not so good. So you can see why omit 很 when using 马马虎虎 makes sense. That's what I think, which hasn't been confirmed if it's the right explanation. But I think that you can take it as a reference. By the way, you should keep 的 in the phrase of 我的汉字. About omitting 的, you can check what I wrote about it here. https://www.italki.com/discussion/104497 I hope my answer helps.

Because "马马虎虎" contains the meaning of "very" -_-||| An adjective word like "马马虎虎" （or other words in forms like "AABB" "ABAB"）, OFFTEN have zoomed the adj. word itself, which means adv like 很，非常can't be followed.

很马马虎虎 is rarely used. It should be “很马虎” or “马马虎虎”

Thanks so much

很 is used to describe how good or how bad it is. We always use 很 together with adj. words like 很好，很差. While 马马虎虎 is a slang or idiom in Chinese,so we don't use them together.