Too strong as cantrip; weak as leveled spell
It's too strong as a cantrip. However, it's also too weak for a first level spell. The reason it's too strong as a cantrip is because there's really no reason not to use it.
Rogues have more things to do with their bonus actions than most classes, but unless you really need to hide or dash, there's literally no reason not to use this cantrip to double your chances to crit, which is stepping on the fighter's toes a bit, because one of their archetypes has that as an ability.
Once you make it a first level spell however, it becomes far too weak, because it's not really worth a spell slot to have a 5% higher chance of critting. The only reason it's too strong as a cantrip is because it doesn't cost you anything, and there's no real reason not to use it. Once it costs spell slots, it's not worth it.
I don't think there is any real way to salvage the spell to make it cantrip worthy.
On bonus action costs
A lot of other answers are pointing out that the cost "isn't next to nothing" because you could be using an off-hand attack, but in all seriousness, what Arcane Trickster is ever going to be using an off-hand attack? You are far more likely to be using Green-Flame Blade or Booming Blade as a cantrip, which means you can't use an off-hand attack.
You can, however, use Lethal Strike and then Booming Blade or Green-Flame Blade.
If you are using the optional flanking rules it'd be fairly trivial to get advantage and double crit chance on a Booming Blade or Green-Flame Blade attack.