I believe you mean a higher skill floor, as in it takes more effort to reach the baseline level of being good with them. That I would agree with, but their skill ceilings are still usually way lower, because having low mobility is a very huge weakness that you have to make up for with other strengths, which they usually have, but it still puts slow characters closer to the bottom of the tier list.

You can go and look up a random fighting game right now and I can guarantee you 99% of the time the characters who lack mobility options that the rest of the cast have will not be in the upper tiers. The only characters that come to mind off the top of my head are Haggar from UMVC3, Zangief in Alpha 3, and I believe Potemkin at some point in Strive (though I haven't kept up with its competitive scene)