Just tell them you don't think they're as good as they used to be. Although this may simply be due to you getting better.
When I first started Aikido, my teachers seemed so amazing, after training regularly for 3 years, going to seminars, trying to regularly train certain things and introduce new ideas I realized that they were.... mediocre. Unfortunate, but true.
There's nothing you can really do to make them be better unless they're interested. Even then they may say they are, but not really be interested.
Not all teachers will be great, either as teachers or as martial artists, but remember you trained with them for 3 years, and that a lot of your progress is due to the time and effort they gave you.
Is not the teacher that helped you build your foundations in the art to be honored as much as the teachers you may have towards the end of your career, when you yourself become great? even when you become better than them ask yourself if you would have advanced in aikido without the first teachers you had?
Would any of us?
They are the ones that kept uscomming back for those first few years, learned what aikido really can be and to differeniate between great and madiocre.
In rideing I've found that I dont really need the greatest teachers on earth to progress. I need a teacher that's slightly better than me, that can spot my mistakes and explain the basics to me. Someone I can talk with and pose questions to. Someone that's a good observer and has the patience to remind me to fix the same mistake again and again untill I actualy remember and the mistake disapare.
I imagine that in aikido it will be the same, it's more important with teachers that can help me stay motivated and judge what mistakes to try to make me fix first, and what to ignore while I struggle with learning where to put my feet and how to move than to have teachers that are the very best of the country's aikido practioners. If I continue with aikido for long enough to someday get good at it, it will be because of the teachers I have now, and their patience and generousity with their time.
I still remember teaching spinning wool into thread to a friend, years ago, and the pride I felt when she became better at it than I was. whenever I teach someone something, that's what I aim for, that my "student" will get better than me, and enjoy it enough that when that point arrive, they'll look for a better teach to take them to the next level.
Without that gift, from someone, it's unlikely that any but the most talented will ever become good. And even a student that some day exeed their first teachers skill, should try to remember that and feel some gratitude for that, even as they move on to better and more advanced teachers. be greatfull that your teachers helped you advance and encouraged you, rather than try to limit you to their own limits.
to the original poste:
If you chose to discuss your teachers skill with them, remember what they've done for you and be polite and respectfull in how you bring it up. studying with someone more advanced than your current teacher doesnt nessesarily mean you need to leave her altogether either.