Nothing wrong with that!. I have done it myself, and I have even enjoyed it. However if you are exploring Spain by car you can easily fit a ferry to Mallorca into your travel plans. And if you are wondering how to get to Mallorca by ferry - I am just about to tell you.
From mainland Spain you can reach Mallorca in between 5 to 7 hours from Barcelona, Denia or Valencia.
From Barcelona you can sail to Alcudia on the North side of Mallorca or to Palma on the west side, while the routes from Denia and Valencia only sail to Palma.
I don't think Barcelona in the north east of Spain needs to be placed further on the map, but if you are not that familiar with Spanish geography you might want to know that Denia is situated on Costa Blanca. A beautiful drive along the coast road N 332 will get you there and so will the highway AP7. Valencia - the third largest city in Spain is approximately 100km further north along the Coast from Denia.
You can book a return ticket, but if you plan to visit one of the other Balearic Islands - may I suggest you book a one-way ticket on the ferry to Mallorca. This gives you the option of sailing back to mainland Spain from Menorca or Ibiza.
You can also reduce the driving quiet a bit if you sail from Denia or Valencia to Mallorca on a single ticket and book your ferry back from Mallorca to Barcelona. - Or the other way around. Doing Mallorca by ferry is a brilliant opportunity to experience different areas of Spain.
Two Ferry Companies covers all the traffic from mainland Spain to Mallorca: Balearia- and Trasmediterranea Ferries. Together the two companies conduct more than 50 crossings weekly.
Taking a ferry to Mallorca from mainland Spain is part of your holiday. Between 5 and 7 hours on the blue Mediterranean Sea, depending on which route you choose, is not a hassle - it is a pleasure.