The results can be pretty good
No. No, they can’t. That program is a common trap for beginners, but the results are worse than normal splices made by hand, at best. The colors are nothing like vanilla GBA, the images are generated in an unusable format (increased size, no mouth/eye frames), and there are loads of little glitchy border areas where stuff doesn’t work right.
Even if making spliced portraits yourself is too hard (which it shouldn’t be - this tutorial is a great place to get started), there are eight Mugging Blitzes full of decent-quality portraits that are free for anyone to use. There’s no shame in taking from the community to cover stuff you can’t or don’t want to do yourself; those mugs were made free for a reason. Just… don’t use that program. Ever.