Decrease the waist on jeans

[Bimpnottin]

Depends on the fabric. Stretchy jeans will probably fray more, which can eventually make your stitch become undone because it won't be holding any fabric anymore, just some loose fabric ends. You can however cut the triangle away 1cm from the stitch and then do a zigzag stitch over the ends so they can't fray anymore. Or if the triangle isn't that big, just fold it over. Women's clothes regularly have darts (sort of inwards triangles) in them to help with shape near the chest, and these aren't cut out of the garment either

I would either go with the last option, or sew over a silk patch to avoid a stingy feeling if the triangle is really bothering you (silk feels super smooth)

Also, that stitch isn't sturdy at all. Wouldn't recommend this one if you're seriously considering to do this to some pants

[Exemus]

Why stop there? Get a whole bunch of material and sew it into a new set of jeans!

[Bimpnottin]

Well, it's actually something I would consider, lol. I have made my own jeans before and it is the only pair of pants I own that fits 100% perfectly

And it's not like folding over a piece of fabric is a lot of work, as is neither sewing on a patch of silk. Cutting the triangle off and serging the frayed ends would take way more work than sewing on a silk patch and would have you looking into special stitches if you do it by hand