I think it depends on what you mean by viable to be honest.
If you intend to compete in tournaments for money and are very serious then you should have any possible option covered and in that case, no it's probably not viable to play as only Harley.
If you intend to play online ranked and vs friends and such, then Id say yes for the most part. I may be wrong here, but I was under the impression she doesnt really have any massively one sided (7-3 or worse) MUs in the game right now.. 6/4s are still very manageable and ultimately skill and ability still counts for more than a 6/4 most of the time. King illustrated this just recently vs Tekken Master.. he picked Batman into Fate, lost and tried to counterpick into Deadshot.. and that quite possibly cost him a LOT.
MUs are figured (or are supposed to be figured) up at the absolute top level of play, so MU charts in descending levels look very different, and change as you get stronger as a player.
I feel like, ifyou want to play only Harley, you'd probably be better off, and you'd win more and git gudder more, than picking up a character you aren't really "into" , and solely have around for a couple/three MUs or whatever in Ranked play where there isn't that much on the line in the first place. The time spent bringing a non-harley from 0-80% or something could be better spent taking your Harley from like 85%- 98% or someshit, if you see what I'm saying. Unless you're a REALLY good already (not saying you aren't I don't know you at all) and don't have much room left to improve - which I'm not sure is a thing in fighting games to begin with.
Ive tended to bounce around and look to other characters to solve some issue I perceive needs solving and the end result has been.. well, let's just call it ineffective. Whats applicable to me may not be to you though.