There is not a simple ‘yes’or ‘no’ answer because there are so many variables. There must be some strong reasons why you are considering the change. Do you like your new role more than your current one? Are you getting paid better or getting compensated via ESOPS? Do you want the excitement of being in a startup and creating something ground-up? Be very clear about why you are thinking of moving. If all these reasons are poweful enough, a lower designation does not matter.
However as a test, think of these two possible scenarios.
First (and I hope this one comes true) you are very happy in your new job and continue with your new company. Then the change in designation doesn’t matter.
The other possibilty is that things don’t work out well and you need to look for another job. Now, this is where designation will matter. Imagine you are attending an interview. Do you have a good explanation for the lower designation? Make sure you can confidently answer the question ‘Why did you go from Account Manager to Account Executive?’
Finally, my advice would be to speak to your new company and ask that your designation be revised, if not immediately, then in a period of 3 or 6 months, based on performance. Put that into your employment agreement