The color blue makes you imagine flowing rivers, water moving forward
Your main issue will be chainline. An mtb bottom bracket isn't what you want, for a couple of reasons.
The main reason for choosing a bottom bracket spindle length is to have the chainrings lined up relatively evenly with the sprockets. An mtb rear hub has 135mm spacing between the locknuts, and a road rear hub has 130mm spacing. So right there you'd want a 5mm shorter bottom bracket spindle (2.5mm shorter on each side) to use an mtb crank with gears that are on a road hub.
But it gets more interesting. On a typical triple crank, the middle chainring is approximately lined up with the middle of the rear sprockets. In your case, you'll only be using the small chainring (and ideally, you'd only have the one small chainring). And so you'd want the inner chainring to line up with the center of the rear sprockets. So you'd want a bottom bracket axle that's 6-7mm wider
on both sides (because you want things symmetrical). So you'd want a bottom bracket axle that's 3.5mm wider on each side (so 7mm wider total) than the axle that ideally fits with the triple while on the mtb. (Chainrings are usually 6-7mm apart.)