Pretend

The first option is the right one!

The first is what most people would assume to be the meaning of the sentence. However, it can technically mean both and is thus an ambiguous sentence. Writers and editors try to avoid sentences like that if the distinction between the two interpretation is relevant enough. In spoken language the distinction is made through stress, body language and other physical cues.

You could try to

PreciousJonesPretend to be nice like my mom.

First, this is in the form of an imperative -- the speaker is telling someone else to pretend to be nice like the speaker's Mom. The speaker is not saying anything about what he himself is going to do.

I'd say the sentence is ambiguous -- it could mean either

Pretend to be nice, like my Mom is.