Not likely for there to be mammals per se, but mammal like species? I think it's very possible on an earth-like planet elsewhere in the universe.
While I don't believe life can only exist on an earth-like planet, obviously earth-like life can only exist on an earth-like planet.
There aren't an infinite number of forms life can take on an earth-like planet.
Physical factors like gravity, atmospheric chemistry, temperature ranges etc frame the possibilities for life forms, and it's likely that within an earth-like frame, earth-like species will evolve.
Life forms that might exist on a gas giant planet (jupiter-like), are never going to be found on an earth-like planet, and visa versa.
Early speculation on extraterrestrial life was certainly earth centric, but the next wave of thinking on the subject was too determined to exclude the possibility of terrestrial-like life forms.
No reason I can think of why there might not be millions of earth-like species populating a near infinite universe.