Did Forrest's mom know about his love for Jenny?

I believe yes. They're obviously close in the film and it seems reasonable to assume they discussed this. Whilst there's no direct evidence in the film to confirm this, Mrs. Gump is certainly aware of Forrest's love for her in the novel. From page 9:

The problem was that wile Miss French was a nice kind lady, the things that she done to me that night was the kinds of things I'd have preferred to have done to me by Jenny Curran. An yet, there was no way I could see to even begin gettin that accomplished cause what with the way I am, it is not so easy to ask anyone for a date.

That is to put it mildly. But on account of my new experience, I got up the courage to axe my mama what to do about Jenny, tho I certainly didn't say nothin bout me an Miss French. Mama said she'll take care of it for me, an she call up Jenny Curran's mama an splain the situation to her, an the nex evenin, lo an behole, who should appear at our door but Jenny Curran hersef!

Also on page 9:

Mama said we can go to the movies an she give Jenny three dollars as we walk out of the house. Jenny ain't never been nicer, talkin an laughin an I am noddin an grinnin like a idiot.

So based on the fact in the books she was aware of his love, it seems likely she was in the film too.