poor作为形容词是
“贫穷的；可怜的；贫乏的；卑鄙的”
非常消极的意思。
因为老外觉得在说英语表达自我的时候，
应该保持自信心，
如果你这时使用poor来表示自己的英语水平不好，
老外只会觉得很突兀，
这么谦虚的吗？
国外知乎上有这么一段分析这个说法的文字，
从语法和文化角度作出了解释
大家可以来看一下帮助理解
First, from the aspect of grammaticality or idiomaticity:
首先，从语法性或惯用性的角度
"My ... is poor" isn't very idiomatic in English.
“My ... is poor”在英语中并不是很地道
"My swimming is poor" well... "I'm a poor swimmer" or "I'm not a very good swimmer" might be better.
你说“My swimming is poor”不如说“I'm a poor swimmer”或者“I'm not a very good swimmer”。
"My speaking skills are poor"... How about "I'm a poor speaker" or "I'm not good at speaking"?
“My speaking skills are poor”...不如说成“I'm a poor speaker”或者“I'm not good at speaking”。
...
In each case, using "poor" as a predicative adjective is not very idiomatic. The more idiomatic alternatives are to say "I'm poor at...", "I'm not good at...", "My ... isn't very good".
在每种情况下，用poor作表语来形容都不是惯用的用法。更地道的说法是“I'm poor at...”,“I'm not good at...”，“My ... isn't very good”。
If you want to include slangy expressions, you might say "My English sucks" --- but if your English was good enough to say that, you wouldn't need to say it at all!
如果你想用一些俚语表达，你可能会说“My English sucks”——但是得是你英语很好才会用到这样俚语表达，侧面也就说明了你根本就不用说你英语不好。
Second, from the cultural or sociolinguistic aspect:
第二，从文化或社会语言学的角度
Telling people your English is poor is just a statement; saying it won't necessarily get the effect that you want.
告诉别人你的英语很差只是一种声明；说它不一定能达到你想要的效果。
If you say "My English is poor" ，you might only succeed in getting people to say "No, your English is fine! Better than mine!", but that's not what you want.
如果你说“My English is poor”，那么你很有可能会成功得到别人回复你说“不，你英语挺好的！比我好！”，但是这并不是你真正想要的效果。
If you want the other person to be aware that you're still struggling with English and to make allowances for that, there are probably better ways of putting it.
如果你想让对方知道你的英语还很差，并体谅一下，可能还有更好的表达方式。
For instance, "Sorry, I haven't been learning English for very long, could you (...say that again please, ... slow down a bit, etc.)" or even "I'm still just learning English".
例如“对不起，我很久没学英语了，你能...（再说一遍...说慢点等等）吗？”或者“我还在学英语”。
If you want to signal that your expression may be awkward or you're not sure if you will be taken the right way, you might say "Sorry, I'm not sure how to say this in English but..."
如果你想表示你的说法可能很尴尬，或者你不确定你是否被正确理解，你可以这样说“对不起，我并不确定用英语如何说...”
And in fact, if your English is really that "poor", people will pick up on it quite quickly. It may not be necessary to say anything at all.
事实上，如果你的英语真的那么差，人们很快就会反映过来。也许根本没有必要说什么。
