His mother was so unloyal

Yes...it helps maintain the suspense of the movie early on -- if your own MOTHER won't believe you, you have nobody.

I like how she laughs out loud in the courtroom when Roger offers his version; how she says "pay the two dollars" at the mansion, and then her coupe de grace:

"You aren't really trying to kill my son, are you?"

There's also a lot of humor here in the Roger/Mother relationship -- how he bribes her at the hotel, how they bicker and banter like a married couple.

Indeed, since Eve Thornhill doesn't turn up until about an hour into the movie -- Mrs. Thornhill is almost like Roger's "female partner" for the first third of the movie, his "date."

There is also this: like so many other Hitchcock mothers, Mrs. Thornhill sure is in Roger's life a lot (the father is likely dead, Roger never seeks him out for help.) It helps explain his two divorces and makes Eve a worthy taker of the throne...a brave, tough woman.

To be fair, his story was very hard to believe, even if it happened to be true.