I have never used mod podge outdoors, but my main concern would be fading. Make sure your decal is made for out door use, like for a car for example. That would mean probably not regular paper. I wouldn't use modpodge on vinyl for example. REgular paper color definitely fades rather quickly out doors, even on a porch. Once you find an out door non fading logo, then you could look for an sdhesive that would work outside.
Another thought is that you could carefully paint over a paper one. That would solve the fading problem.
good luck
Kathy