I've also thought about this match often and wondered whether JA means for us to see some irony in the situation.

Here we have Mary, who is serious in an odd, absurd way and vain about accomplishments she hasn't really achieved. (She really thinks she's a superior musician, right?) She also seems to think she's standing atop the moral high ground, when really she's just aimlessly kicking fine words around. (Her comment about the olive branch always cracks me up!) She's plain, impatient for display (craving attention?) and gives the impression of being socially inept. Her chances of marrying at all--let alone marrying well--seem very slim.

Along comes Mr. Collins, who could set a world record for absurdity. He's so anxious to please that he practices flattering ladies in front of the mirror--a practice that seems to have backfired and given him a high opinion of himself. ;-) He thinks that being a clergyman makes him as a good as a king, so long as he's properly humble about it; he gives offense everywhere he goes but fancies that he's uniformly charming. He pays lip service to seriousness and religion, but the only devotion we ever see in him is to his noble patroness, Lady Catherine de Bourgh. ;-)

So far the resemblance between Mr. Collins and Mary appears very great, but there is in fact a great gulf between them. (IMHO, of course!) :-) While Mary seems doomed to spinsterhood, Mr. Collins has a much greater chance of finding a mate. His manner of selecting from among the Bennet sisters--a circumstance which would make an interesting topic for discussion in itself, IMO--is an excellent example of this.

He begins by choosing the eldest and most beautiful Miss Bennet and when she turns out to be unavailable, he turns without blinking to the next in line. Although we know Elizabeth would no more accept him than she'd drop in at Netherfield for a cozy chat with Caroline Bingley, he thinks it's a perfectly reasonable application. What's more, her mother agrees. Even though the judgement of both Mr. C and Mrs. B is clouded, this does show us that he has some room for choice; there are women who would set aside his absurdities and accept what he has to offer--a comfortable home and the promise of an even more comfortable one in the future.