In math, dedications are rare but not unheard of. They are most common in birthday conference proceedings, or special issues devoted to summarizing a person's life work. For example, consider the following papers that have a birthday dedication:
Gunnells - Robert MacPherson and arithmetic groups
Nollet and Schlesinger - Curves on a Double Surface
Mulase and Zhang - Polynomial recursion formula for linear Hodge integrals
International journal of Numerical Analysis and Modeling, volume 15, number 4
Kinoshita, Power, and Takeyama - Sketches
International Conference On Logical Algebras and Semi-rings
I found these and dozens more by searching Google for "math" "dedicated to" "occasion" "birthday". Similarly, there are plenty of examples dedicated to the memory of a mathematician who recently died ("math" "dedicated to" "memory of"):
Rowell - An Invitation to the Mathematics of Topological Quantum Computation
Differential Equations and Applications, volume 10, number 1
Propp - Exponentiation and Euler measure
Discrete and Continuous Dynamical Systems - Series B, volume 21, number 2
People have also dedicated papers to non-mathematicians, e.g., Chris Kapulkin dedicated this paper to his mother.
There is ample evidence that the journal will not normally object to a dedication. I have even seen papers dedicated to the memory of Grothendieck, by authors who are unlikely to have known him simply because of the large gap between when he left mathematics and when these papers were written. It's clear that someone's work could have a big impact on a young researcher, even if the two never met. I would say it's "not inappropriate" to write a dedication in such a situation, even if it's rare.
In the papers above, you see some dedications right under the author name and above the abstract, and you see others that are a sentence in the acknowledgments. For a junior author dedicating a paper to a senior mathematician they never met, I'd err on the side of putting it in the acknowledgments so you have more space, to write a full sentence about how that person's work touched your life. The short style above the abstract of simply "This paper is dedicated to X" might raise a question in the mind of the reader, though, to be honest, I think most people would ignore the dedication regardless of where you put it. Lastly, there were two threads on academia.SE about dedications:
