As far ai I (we?) know Con, "bm" means bowel movement.
I saw another person use it; when I asked them what it meant they said it stands for 'bookmark'. I placed that very short post there so that this thread would appear in my "My Posts" list. This was because I saw the question while I was browsing the web on my phone, and I did not fancy doing much typing, especially as the bus ride was rather bumpy. I can forgive many things, especially puns, especially when (refreshingly) the perpetrator is also demonstrating that he knows the difference between "passed" and "past". (Many younger folks don't, I have found).
Now to the question:
1. Sales of cars dropped by 3 percent, reflecting a somewhat weaker demand than expected.
The first sentence is natural and grammatical.
2. Sales of cars dropped by 3 percent, reflecting a somewhat more weakly demand than expected.
The second sentence "sounds unnatural", and this is because it contains a serious and glaring grammatical error. The word "weakly" is an adverb, and cannot be used as an adjective as it is in that sentence. You could use "more weak", but that uses one extra word for no good reason.