Is it permissible to give charity on behalf of my mother instead of returning the money to the hotels from which she took free food, especially if reaching them is not feasible?

What your mother did would be forbidden if she took food from the hotel without permission, and the hotel only allowed consumption on its premises. However, if it explicitly or customarily allowed taking food out, or if she took it with permission, then it is not forbidden.

If what she took was forbidden, she must seek absolution from the hotel owners by asking for their forgiveness or by returning the value. Charity is not sufficient unless she is unable to reach them.

If you wish to voluntarily return the right on behalf of your mother, this is good and will suffice for her, clearing her liability. This applies even if it is without her permission, just as paying off a debtor's debt without their permission is valid.