Unfair advantage

I've had some sit-down thought on this.  First of all, Superman's time-travel abilities are, as some have described, more of an extrapolation on the concept of super-luminal flight that he was considered to have.  Second, Superman has always had what, for such a simple concept of a flying break-house with energy attacks, amounted to a "Whatever the author needed at the time" tendency with his powers.  He was superman, so he could do anything (You know, when they weren't trying to lower his power and give him weaknesses like Magic for thematic purposes or to make Batman look cooler.).

Ultimately, as some have said:  This is your character.  If you believe that she should be written with the ability to travel in time then do so.  When you do so, just be clear how she does it and if it weakens her afterwards.  Then be clear how you plan to handle time travel's impact:  Is the present predetermined, or does her actions in the past result in alternate realities.  You don't necessarily need to tell us as readers.  Just know for your own purposes so you can keep your story straight.

In terms of "overpowered," I doubt that this is a matter of overpowered even if detractors use the term.  For all that Time-Travel sounds extremely impressive, it is thematically a very limited ability with extremely limited functionality.  In a knock down dragout fight, it does nothing.  Literally nothing unless she can flash back a few seconds to dodge an attack, and it's just as easy to have her simply dodge the attack the old fashioned way.  Rather than an "unfair advantage," what it is felt by some to represent is an "out of type ability" with significant potential impact.  For example, take the Flash:  Nobody bothers to give him Telekinesis or Mind Control because, while these powers don't necessarily kick him over the edge in abilities (Let's be honest, neither would benefit the Flash in his usual MO during a fight.), they are very much "outside" of what one expects with a traditional "Runs faster than the speed of sound" Speedster. 

Does this qualify with a Superman-analogue character like Superhawke?  Eh, that's a debate that could blow a few needless essays on the subject of the speed of light, Superluminal speeds outside of the Earth's atmosphere and the like.  I wouldn't say it's "Out of Type."  It's certainly at the edge of Superman as a type, ignoring the film (which took a lot of liberties), but it's also something they've toyed with before.  The question, though, to me is less "Can you do this," and more "Why do you want to do this?"

If it's simply a power she has, but one you never want to use in a fiction, then the thematic question exists:  What is the purpose in including it for her?  If you're planning to use it, however, go for it.  Even if it's only a one-time deal, it can be a fairly fascinating one that comes with massive moral dilemmas over whether she can be justified in changing the past, and what would make her even consider it.