Not so good
I think it's quite bad. Mario, the only character in the game, has only four or five sprites, the background is only one picture, the only thing you can do is run and jump, and you have only three enemies, the boss included. Another bad thing is that it's too short. I beat it in only 40 seconds. You can improve all of these things. Also, make a use for the coint you can collect. Make a shop so you can buy boots that lets you jump on enemies or buy health bonuses, or at least make a medal for getting all 115 points.
And for the people who can't defeat the boss, touch the heart on the back side for about two seconds.