Even the actors who have played it a bit more seriously like Pertwee, Hartnell, and Davison did their share of comedy.
I haven't yet seen anything of the Davison era, and Hartnell certainly had a more serious approach to the role, but Pertwee's Doctor wasn't really seriously played. He always has time for banter even in the most dire circumstances. His costume already doesn't exactly emanate seriousness. Also, Pertwee was mostly a comic actor and I'm pretty sure the producers of the series had that in mind when they hired him. He played the role a lot less openly comic like Troughton before him, that's true, but he's certainly not on the same level of seriousness as William Hartnell.
No, he was playing the role seriously in fact.
http://drwhointerviews.wordpress.com/category/3rd-doctor/
Q: I mean up to the time that you played Dr. Who, the characters you played were loud, were humerous. Was ‘Who’ the first time you played straight?
JP: It was, really, because I remember Shaun Sutton, who was the head of programmes at the time and a dear friend of mine, he asked me to do it, I said ‘Well let’s have a bite of lunch’, we had lunch, and at the end of lunch he said ‘Well will you play it?’, I said ‘Let me think about it’, he rang me up the next week and said ‘Do you want to do it?’, I said ‘How about another lunch?’, we had another lunch and he said ‘So, do you want to do it?’, I said ‘How about dinner next week?’. After about three meetings I said ‘Well I’ll play it, but how do you want me to play it?’. He said ‘As Jon Pertwee’, and I said ‘Well who the hell’s that?’, because I didn’t know who Jon Pertwee was, I’d never played myself, I’d hidden under a green umbrella all my life, like Peter Sellers, my friend Peter Sellers used to do…