It's easy to get a player to ride a mech in Construct. Both need to have the Platform behavior. At the start of layout, deactivate platform movement for the mech.
Next, set an event for when the player touches the mech (say, on collision between player and mech). In a nutshell, the actions for this event should be as follows:
- Deactivate platform movement for the player.
- Make the player sprite disappear.
- Activate platform movement for the mech.