I want to ask my girlfriend to stop using such thick lotion
Why would you ask that? Is it harming you in some way?
Yes, I get irritated by the smell
It's okay for you to ask your girlfriend not to use so much lotion, but let her know that you are sensitive to the smell, and not that it is her fault in any way. However, if she wants to keep using it, that is her decision to make, and you do not have the right to tell her how to treat her own body.
Okay, hopefully she will change it.
If she cares about you she will. It's not a big deal.