Bunny senpai? Kinky.
Jokes aside, your interpretation of ってくらい being というくらい is spot on. This phrase is made up of two parts: という and くらい. If you split up the sentence into these two sections it might be easier to understand.
耳から溶け出すんじゃないかって
going to melt and fall out out of my ear.
The って here just quotes the previous clause. という can be used to modify the noun that comes after it, which in this case is くらい, which means extent. So if you add it to the end:
耳から溶け出すんじゃないかってくらい
To the point where (it) would melt and fall out of my ear.
If you put the two clauses:
脳みそが柔らかい。
My brain was soft.
耳から溶け出すんじゃないかってくらい
To the point where (it) would melt and fall out of my ear.
together, you would get:
脳みそが耳から溶け出すんじゃないかってくらい柔らかい
My brain was so soft that it felt like it would melt and fall out of my ear.
I hope this helps!