Yes, it probably is bad. MySQL's query planner is pretty much of the variety that if you give it an index it will likely use it, whether it makes sense or not. Generally you want to use indexes on fields that are highly selective (i.e. have a lot of variety and when you choose based on a single value you'll get a tiny part of the table).
With a two value enum you've got a field that is quite likely to get you 50% or more of a table each time you access it. An index won't generally do you a lot of good here.
Now, sometimes, making a compound index on a field like that can make sense, because the compound index might be much more selective. especially if you're going to choose / order by those two fields a lot.
