Explanation
No. Medieval plate armor made for battle did not normally prevent its wearer from mounting a horse. The familiar image of a knight being lowered into the saddle by a crane misrepresents its mobility.
The Metropolitan Museum of Art describes historical illustrations of armored riders mounting without lifting equipment. Royal Armouries independently explains that plate armor distributed weight across the body and cites an early-fifteenth-century account of an armored man vaulting onto a warhorse.
Some specialized tournament armor was substantially more restrictive than battle armor. Assistance from a squire or a mounting step should not be confused with needing a crane.
Key takeaway
Battle armor allowed practical movement. Specialized tournament equipment should not be treated as typical battlefield armor.